Answer:
55 mph
Step-by-step explanation:
Speed = Distance/Time so Time = Distance / Speed.
If the first speed is x mph, the first trip will take 660/x hours.
The second trip will take 660/(x+5) hours.
We want to find x such that (660/x) - 660/(x+5) = 1 (i.e. solving this equation for x).
Multiply both sides by x*(x+5):
660*(x+5) - 660x = x*(x+5)
660x + 3300 - 660x = x2 + 5x
x2 + 5x - 3300 = 0
(x - 55)*(x + 60) = 0
So either x = 55 or x = -60. Since x > 0, x = 55mph.
