Mike travelled from Calabasas to Palo Alto at an average speed of 60mi/hr.

Two days later after visiting Stanford University, he drove back home at an

average speed of 90mi/hr. What was Mike's average speed for the entire 

trip?

 

a. 70mi/hr

b. 75mil/hr

c.  68 mi/hr

d. 72 mi/hr

e. Not enough information