Well here's the solution to the little brain teaser that I gave a few days ago. The first thing you want to do is figure out the equation to solve:

A + B = 91
A = 3(B - x)
2B = A - x

The key here is really the A-x, B-x parts. We know there's a comparison of ages in the past, but we don't know when in the past that is. x will tell us this. So, first we need to express A and B in terms of x and then solve for x. Substitute A for B first:

2B = A - x
2B = 3(B - x) - x
2B = 3B - 4x
-B = -4x
B = 4x

Then substitute B for A next:

A = 3(B - x)
A = 3(4x - x)
A = 12x - 3x
A = 9x

Now substitute x for A and B and solve for x:

A + B = 91
9x + 4x = 91
13x = 91
x = 7

Now it's just a matter of substituting 7 for x and getting the answer to the original equation.

A = 9x
A = 63

B = 4x
B = 28

A + B = 91
63 + 28 = 91

Thus Ann is 63, and Bill is 28.