Question 958255
10 mi/25mph=0.4hr He has spent 0.4 hr going 10 miles
20mi/40mph=0.5hr To make the 20 mile trip at an average of 40 mph, he must complete the trip in 0.5 hour. He has already used 0.4 hour, so he must travel the last  10 miles in 0.1hr
10 miles/0.1hr= 100 miles per hour.
ANSWER: He must complete the trip at 100mph to average 40 mph for the whole trip.