Arecreation 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.