laura has $87 in one dollar bills and five dollar bills. She has three more five dollar bills than one dollar bills. How many of each does she have?
Let N be the number of one dollar bills.
Let F be the number of five dollar bills.
Then,
N*1 + F*5 = 87 (this is all the money she has)
F = N + 3 (Number of five dollar bills is 3 more than the one dollar bills)
Substituting for F = N + 3 into the other eqn above,
N + (N + 3)*5 = 87
6N + 15 = 87
6N = 72
N = 12
F = 15