Answer: 60 mph
Explanation:
Given
Truck takes [tex]1.5\ hr[/tex] to make [tex]90\ mile[/tex] trip
The average speed is the ratio of total distance traveled to the total time taken.
[tex]v_{avg}=\dfrac{\text{Distance}}{\text{time}}\\\\v_{avg}=\dfrac{90}{1.5}\\v_{avg}=60\ mph[/tex]