SOLUTION: Tom is 2 years older than Sue and Bill is twice as old as Tom. If you add all their ages and subtract 2, the sum is 20. How old is Bill?

T = S+2            <--first equation

B = 2T               <--second equation

T + S + B - 2 = 20   <--third equation

Use the first equation to substitute (S+2) for T in the second equation.

B = 2(S+2)     <--fourth equation

Use the first equation to substitute S+2 for T in the third equation.
Use the fourth equation to substitute 2(S+2) for B in the third equation

  T   + S +    B   - 2 = 20   <--third equation
(S+2) + S + 2(S+2) - 2 = 20
S + 2 + S + 2S + 4 - 2 = 20
                4S + 4 = 20
                    4S = 16
                     S = 4, so Sue is 4 years old.

Substitute 4 for S in the first equation:

T = S+2            <--first equation
T = 4+2
T = 6, so Tom is 6 years old.

Substitute 6 for T in the second equation:

B = 2T             <--second equation
B = 2(6)
B = 12, so Bill is 12 years old.

Bill is 12 years old.

Edwin