The attached data sheet is very typical ; because a*x does not equal to b in real practical scenario ;
actually, this b answer is nothing but we can say a1x1+b1x2+c1x3+.....+t1x20 = b ;where b is known, a1,b1,...t20 is known but x1,x2 ,x3 are not known. like that equation is there.
similary...