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Singapore Birthday Riddle

Singapore Birthday Riddle

(OP)
This one went around the internet awhile back. But, I saw it again on MSN today with the wording cleaned up to make the assumptions a little more obvious.

Albert and Bernard just met Cheryl. “When’s your birthday?” Albert asked Cheryl.

Cheryl thought a second and said, “I’m not going to tell you, but I’ll give you some clues.” She wrote down a list of 10 dates:

May 15, May 16, May 19
June 17, June 18
July 14, July 16
August 14, August 15, August 17

“My birthday is one of these,” she said.

Then Cheryl whispered in Albert’s ear the month — and only the month — of her birthday. To Bernard, she whispered the day, and only the day.

“Can you figure it out now?” she asked Albert.

Albert: I don’t know when your birthday is, but I know Bernard doesn’t know, either.

Bernard: I didn’t know originally, but now I do.

Albert: Well, now I know, too!

When is Cheryl’s birthday?

RE: Singapore Birthday Riddle

Old logic puzzle... July 16 as a WAG.

Dik

RE: Singapore Birthday Riddle

It has to be either May 19 or June 18 since those are the only unique days.

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA
Siemens PLM:
UG/NX Museum:

The secret of life is not finding someone to live with
It's finding someone you can't live without

RE: Singapore Birthday Riddle

(OP)
Incorrect....

If the day were unique, then Bernard would have known immediately without Albert having to say anything. Right?

RE: Singapore Birthday Riddle

Exactly what JoshPlum says. Bernard /would/ know immediately if it were either of those. Meaning it cannot be, since Albert knows Bernard cannot immediately know.

Spoiler-tagging the rest:

Spoiler:

So this means that May and June are not possible MONTHS for the birthday. All Albert knows is the month. Since he knows the date cannot be May 19 or June 18 (else, Bernard COULD know already) then we know those months are not possible.

Thus we are left with either July or August. Bernard knows it will be either of those.

At this point, Bernard declares he knows. So the date cannot be a date repeated in two remaining months, thus not the 14th. So we're left with Aug 15, Jul 16, Aug 17.

Albert declares that he, too, now knows. Thus it cannot be a month with two possible dates left. The only viable option is July 16.

HOW Albert knows, is because if he were told August, he would still be uncertain. Bernard could have been told either 15 or 17. If Albert was told August, he would not be sure. For him to be sure, he must have been told July.

/edit/ I initially wrote I was confused about how a certain logical leap was mode. Edited because it finally clicked, and I explained.

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