What must the average be on the last part to complete on time?

Question

Evan is on the 24 mile bike trip. He wants to average 10 mph. He is halfway to his destination and has average 8 mph. What must his average be on the last part to complete on time?

Answer 1

Distance=speed times time. Time=distance/speed. Speed=distance/time.

To average 10mph for the whole trip he must complete it in 24/10=2.4 hours.

He has completed 12 miles at 8mph and this has taken him 12/8=1.5 hours so he must complete the remainder of his trip (12 miles) in 2.4-1.5=0.9 hours. His speed must be 12/0.9=120/9=40/3=13⅓ mph.