Answer: 64.8 mph
Step-by-step explanation:
Given
Arriving time is 6 hours
return time is 4 hours
The average speed of returning 81 mph
Distance traveled while returning
[tex]\Rightarrow d=81\times 4=324\ miles[/tex]
the average speed of the entire trip is
[tex]\Rightarrow v_{avg}=\dfrac{2\times 324}{6+4}=\dfrac{648}{10}=64.8\ mph[/tex]