Answer:
36 mph
Step-by-step explanation:
Let
D be the distance covered in on trip
x be the rate on return
y be the going rate
if the Jones family drove to Kankakee in 5 hours and returned in 4 hours.
y = D/5 and x = D/4
D = 5y and D = 4x
5y = 4x -------Equation 1
Their rate on the return trip was 9 mph faster than their rate going
x = y + 9 -------Equation 2
substitute into equation 1
5y = 4(y + 9)
5y = 4y + 36
collect like terms
5y - 4y = 36
y = 36 mph
