×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Brain Teasers
2

Brain Teasers

Brain Teasers

(OP)
I've got an interview with a company tomorrow that's notorious for asking difficult brain teasers.

Do you have any such questions that you remember from any of your interview experiences?

RE: Brain Teasers

Such questions are not asked with the intent of getting the right or wrong answer.

They are asked to see if you have, and can articulate a logical thought process.

RE: Brain Teasers

I'm sure a search of this forum would yield many similar threads. I remember reading a few of them.

Not trying to be rude, but there is a lot available on this site in regards to this topic if you search for it.

RE: Brain Teasers

Whip out your internet-connected smartphone and Google the answer.

RE: Brain Teasers

(OP)
I wasn't asking so I can be prepared with the answers, nor was I looking for a list of technical brain teasers, which are readily available on the internet. I wanted to know if any experienced engineers had come across an interview question they thought was particularly interesting, challenging, or illustrated the idea of a brain teaser, that is, to assess one's logical reasoning skills in a notable way. I posted my question here because this is an engineering forum, not a computer science one like most of the brain teaser sites are geared towards, as this has been popular in tech for a while.

RE: Brain Teasers

"
A particular critical part's drawing carries a high limit for the number and volume of detectable voids.
...
What would be the maximum number and volume of undetectable voids?
"

Mike Halloran
Pembroke Pines, FL, USA

RE: Brain Teasers

We had a problem like that in high school: how many molecules of a drop of rain from the Jurassic wound up in my canteen today? It was fun; the same problem showed up in freshman physics and drove a few people crazy for a while.

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

RE: Brain Teasers

My favorite twos:

-How do you make something smaller?

-How do you define engineering?

RE: Brain Teasers

"-How do you make something smaller?

-How do you define engineering? "

Probably a great way of finding out how sarcastic I'm feeling that day, maybe not so good at finding out how I think.

I like hammers, for making things smaller, and engineering is knowing which bits you can knock off and the thing will still work.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Brain Teasers

Greg

My first reaction was with an axe, then I thought about it being steel.

Mike

Total volume of undetected voids is easy by apparent vs real SG after you account for detected voids. Number well not so easy other than the total volume vs minimum detectable size as a minimum number. I guess "who cares" would not go down well.

Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
http://eng-tips.com/market.cfm
for site rules

RE: Brain Teasers

How do you make something smaller?

Shrinkage.

I saw it on Seinfeld.

RE: Brain Teasers

"Who cares?" is a correct answer. The voids are not detectable by ANY means including SG and part failure.

Mike Halloran
Pembroke Pines, FL, USA

RE: Brain Teasers

Mint is right - they are really looking for you to showcase your reasoning ability to find a logical answer. While not a brain teaser, a likely interview question that I've encountered before is "What is your greatest weakness"? I answered it by saying, "I have a low tolerance for incompetence". Be prepared for that one, since it may come up.

Maui

www.EngineeringMetallurgy.com

RE: Brain Teasers

If the voids are undetectable, the maximum number possible is infinity.

RE: Brain Teasers

Describe the universe and give two examples.

RE: Brain Teasers

How high is up??

RE: Brain Teasers

TenPenny - I disagree. You can't really call something a "void" unless there is actually extra space there beyond the atomic spacing in a perfect crystalline structure.

I would think minimum void size would be on the order of one atom missing from the crystal lattice. You would also need a one-atom thickness between voids, so each void needs to be surrounded on all faces by atoms. Argue that those atoms are "shared" between voids, make an assumption about crystal structure, and a ratio of atoms to voids (perhaps 3:1). Take the volume of the part and atomic spacing, calculate maximum theoretical number of voids. Max volume would be 1/4 of part volume (adjust if you don't agree with my ratio.)

RE: Brain Teasers

Got asked this in an interview, almost crapped my pants:

There are three men sitting in a row looking forward, each man can only see in-front of them and cannot see what is on their heads.

A box has three red hats in it and two blue hats, these hats are put on the men at random.

The man in the back is asked if he knows what color hat he is wearing, from observation of the men in front of him, he says no.

The man in the middle is asked if he knows what color hat he is wearing, from observation of the man in front of him, he says no.

The man in the front is asked if he knows what color hat he is wearing, from inference, he says yes.

What color hat is he wearing and how does he know?

RE: Brain Teasers

If a man talks in a forest where there are no women around to hear him, is he still wrong?

RE: Brain Teasers

The "solution" to the hat puzzle assumes that the first two men that don't know the color of their hats don't know based on the logical deduction process described. There is no reason to believe that they are smart enough to reason that out.

RE: Brain Teasers

Mike, are you saying the mind control waves come from that pile of junk downtown?

RE: Brain Teasers

I was asked this tricky math problem.

A ship has a ladder with 12 rungs above water during the low tide. How many rungs are above water during high tide 24 hours later, if the rate of rise of water level is 0.5inches per hour and the rungs are spaced 4 inches apart.

RE: Brain Teasers

It's a poorly worded question "trick" for a variety of reasons:

Low tide and high tide are not 24 hours apart. They are roughly 6 hours apart, but not quite. Do you happen to know this fact? Did the writer?

So, if we presume they meant 24 hours later, 12 rungs will be above the water. If they meant the next high tide, we get 3 inches, so either 11 or 12 rungs will be above the water, depending on the (undefined) start point relative to the lowest rung.

Rate of seawater rise is nonlinear during those 6 hours.

Tide height can vary significantly from one high (or low) to the next.

...and the most likely intended answer: ships often (but not always) will float with the tide, leaving us 12 rungs anyway. (hey, if I put that first, it would give away the intended answer.)

One of the "not always" examples would be the USS Enterprise aircraft carrier (retired) in Corpus Christi, TX - it rests on the bottom and does not float with the tide.

Rare example you say? Sure, lets go with reality then:

Most likely the ship will have a change in its weight of ballast, fuel and/or cargo thus changing the depth. Precious few ships sit around in the water with no change to weight over a 24 hour period. Maybe you have a persnickety captain who always trims to within a few inches - very unlikely.

RE: Brain Teasers

TomDOT

We could suggest that if high tide is 24 hours after low tide, the ship must be somewhere else, so we'd have to calculate how much fuel was used to travel that far, in order to figure out the change in freeboard.

RE: Brain Teasers

Of course if that ladder is from the main deck to the superstructure then all 12 rungs are above water regardless of tide, freeboard, draft, weight....

RE: Brain Teasers

A question I heard in a movie:

If you have three apples in one bag, and two apples in another bag, how many bags do you have?

RE: Brain Teasers

jmcoope3, the man in the front is wearing a red hat.

The man in back could only know what his hat color is if the two men in front of him were both wearing blue hats. Since he doesn't know, at least one of the men in front of him is wearing a red hat. Since the second man can't tell what hat color he has on either (it has to be either red or blue), and we know based on the first man's reply that at least one of the remaining two men is wearing a red hat, this indicates that the man in the front is wearing a red hat. Make sense?

Maui

www.EngineeringMetallurgy.com

RE: Brain Teasers

If the man in front was wearing a blue hat, the guy in the middle would know his hat is red. If the man in front was wearing a red hat, the guy in the middle would not know if he is wearing a red hat or blue hat. Since the guy in the middle did not know the color of his own hat, the front guy has on a red hat. Both could be wearing red hats.

RE: Brain Teasers

So where has our OP gotten to?

How was the interview and what question was asked?

RE: Brain Teasers

Situation: your coffee is hot and your cream is chilled but you haven't mixed them yet. You won't be taking your first sip for 30 minutes, and you would like your drink to be as hot as possible when you begin enjoying it. Should you mix the coffee with the cream right now, or wait until you're ready to drink it?

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

RE: Brain Teasers

coffee...
wait.

That hat thing has got me...
if person one and person two have no information. then person three is guessing.

ship...
if it's 24 hours later.. then it would not be high tide it would be low tide, the situation has not changed.

I suppose i'm under thinking all of it

RE: Brain Teasers

Not guessing, logic. In each of the 7 possibilities, one person can figure out, the last guy should get it 4 of the 7 possibilities, the middle guy 2 of the 7, and the first guy only 1 of 7, which is the case where he sees both of the blue hats. When the first guy sees any other possibility, he cannot determine his own color. In the cases where the middle guy sees a blue hat, he knows his is red, which occurs twice. The remaining 4 possibilities always occur with the last guy, who can't see any hats, wearing red. It's kind of an interesting problem.

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

RE: Brain Teasers

I was actually asked how a Newton's cradle worked once.

- Steve

RE: Brain Teasers

By the way, I answered the coffee teaser with:

For the hottest coffee thirty minutes from now, mix them whenever you want and nuke it in the microwave for a minute when you're ready to sip.

The thermo calculations can't be done anyway, there are too many unknowns.

I got a big smile from the interviewer. It might have been a sarcastic smile, though. I didn't get the job offer.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

RE: Brain Teasers

Heat loss (from the coffee/cream system) is primarily proportional to the temperature difference from the ambient environment. Mixing the two immediately will lower the rate of heat loss and result in a higher temperature compared to mixing later. The hot coffee is at a greater difference in temperature from ambient than the chilled cream so there is no advantage to waiting so the cream can warm. If you get hung-up on not knowing enough details, then perhaps you are not the type of engineer that is being sought.

RE: Brain Teasers

Composite -- you missed the coffee portion of the waiting. It's in a heated coffee pot.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

RE: Brain Teasers

[dadgum arthritis -- poor mousemanship sometimes means a too-quick "submit post"]

If the coffee's in a heated pot, all the heat it loses will be replaced during the half hour. Wait and pour it then.

Run some tap water or hot water over the cream container in the breakroom sink. Give it some extra heat to start with.

Etc...

In other words, you're right, Compositepro, in one sense. If he were looking for an engineer who knows his or her stuff, and knows when to stop digging for data, your answer gets the job.

I would personally prefer to hire the engineer who suggests methods that would improve the outcome over the two less-than-optimal methods he was given to evaluate. I would also prefer to work for someone who expects questioning.glasses

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

RE: Brain Teasers

Coffee question:

I'll make (and state*) for the interviewer some environmental assumptions: We have no external heat inputs available, there are two containers (one for cream, one for coffee, and that we have significantly more coffee than cream. The coffee will be significantly above ambient temperature, the cream will be moderately below ambient temperature. These are the "best fit" of likely conditions - coffee around 190C, cream around 2C, environment around 20C.

Immediately add the cream to the coffee, then place the cream container on top of the coffee cup to arrest evaporative cooling and reduce convective cooling.

Ha!

*Throw in a question back, such as "Okay, is that the limit of the data I have on hand or can reasonably obtain?" to make sure it isn't one of those "lets see how you research" type of questions.

RE: Brain Teasers

TeamDOT,

How does one obtain 190C coffee at atmospheric pressure again?

RE: Brain Teasers

fegenbush: Clearly you have never experienced the sort of coffee they make in my office.

"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." -Scott Adams

RE: Brain Teasers

That remindes me of my one trip to Spain.
Got to the hotel breakfast buffet, and poured a cup of coffee and a touch of hot milk.
The coffee was terrible, but I needed a second cup regardless.
This time I was behind one of the natives, and noticed he poured about 1/4 cup and the rest milk. I followed suit, and behold, a latte! The reason for the hot milk.

RE: Brain Teasers

fegenbush: Please remove the leading 1 and enjoy some 90C coffee :)

RE: Brain Teasers

Ease up on the guy - he's allowed to feel a bit of pressure under interview conditions.

A.

RE: Brain Teasers

My favourite brain teaser is the one where you are sitting in a boat in a small pond, holding a brick. Your throw it overboard. What happens to the water level in the pond?

- Steve

RE: Brain Teasers

That's pretty much what I told the aggressive, smug interviewer who asked me that question. He said I was wrong and then gave a wrong answer himself (claimed that for the water level to stay the same, the brick would need a relative density of exactly 1).

I was 17, hundreds of miles from home, desperately needing to find a sponsor and quite intimidated by the panel of 3 interviewers. So I didn't argue the case. Or get the job.

- Steve

RE: Brain Teasers

Here’s another one. How do you calculate the cooling rate of an insulated room after you open the door of a refrigerator?

I've got a PHD is Broscience

RE: Brain Teasers

Trick question. You don't sit and dawdle and daydream with the refrigerator door open. What are you doing, trying to air condition the great outdoors?

RE: Brain Teasers

Check the wattage label on the refrigerator, measure the volume of the room, figure how many calories you eat in a normal day & convert to watts, add together for total heat released to room in joules, use the specific heat of air (just for simplicity's sake) and the volume of air for temperature increase over time. (Presumably, the air inside the refrigerator counts in the initial estimate of the room's temperature.)

Unless you mean the cooling rate of the room, in which case you'd need to measure the efficacy of the insulation and the difference between the inside/outside temperature.

"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." -Scott Adams

RE: Brain Teasers

nech - I'll presume the refrigerator is entirely within the otherwise sealed room with nothing else.

Cooling rate is negative - fridge is heating up the room. Start with average power draw by the fridge, convert to heat. Then go forward with EngineerErrant's calcs.

RE: Brain Teasers

I was going to answer, but I thought it might be a trick question. There will be an initial transient as the two control volumes become one. That's before the steady heating effect as the thermostat in the fridges keeps the thing running. Assuming the thermostat is a two-state device, heating rate should then be constant, depending on the power rating of the fridge.

- Steve

RE: Brain Teasers

Sompting- Back to the displacement question, I see the smug interviewers statements as correct. If you have a bucket of water in a boat and pour it into the pond the water level is the same, the volume of boat rising above water level is the same amount as water added. If it is a brick the water level in the pond goes down as the volume of the boat rising out of the water is greater than the volume of the brick added to the pond. Or maybe I missed something?

Comprehension is not understanding. Understanding is not wisdom. And it is wisdom that gives us the ability to apply what we know, to our real world situations

RE: Brain Teasers

If the SG (which is unspecified) of the brick is greater than 1, while in the boat the weight of the brick is displaced as part of the boat.

Once the brick is removed it sinks and displaces its volume in water which is less than its weight.

If the brick is SG 1 or less it displaces its weight in water either way so no change.

Unless otherwise specified the definition of a brick is a ceramic or earthenware object used as a building material. these bricks always have an SG of more than 1.

The interviewer was less than justified in being smug, however proving that probably goes against you and reinforces "lifes not fair".

Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
http://eng-tips.com/market.cfm
for site rules

RE: Brain Teasers

Unplug the fridge, open the fridge door. Room temperature drops. talking of which when are we going to get domestic fridges with remote radiators? I'm sick of sharing my house with a 400++++ W heater in summer.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Brain Teasers

Greg

I had an onsite caravan where I cut a hole in the wall the same size as the fridge. Put the fridge in with the condenser exposed to the outside then sealed the sides of the fridge to the wall of the van. Worked for me as we only used the van in summer.

I put some strategically placed trellis work to protect the condenser from passers by and vise versa.

Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
http://eng-tips.com/market.cfm
for site rules

RE: Brain Teasers

Displacement question. A simple thought experiment requiring no laws of physics.

1) I have a big brick of gold in the boat. The boat is close to sinking due to the huge weight of the brick and the water level in the pond is close to overflowing its banks. I throw it in. My boat is suddenly unladen, so the water level drops massively. When everything has stopped bouncing around, the level is slightly higher (volume of gold brick) than it would have been had there been no brick to start with. But lower than it was when the boat was thinking of sinking.

2) My brick is expanded polystyrene. It floats nicely on the pond. Nothing changes with the water level.

- Steve

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources