A recreation center charges nonmembers $3 to use the pool and $5 to use the basketball courts. A person pays $42 to use the recreation facilities 12 times. How many times did the person use the pool?

Let x = times in the pool

Let y = times in the basketball courts

42=3x+5y

x+y=12

Therefore y = 12-x

Subsitute for y

42 = 3x + 5 (12-x)

42 = 3x + 60 - 5x

Subtract 60

-18 = - 2x

9 = x

The person used the pool 9 times.