Expert: Richard J. Raridon - 10/20/2004

Question
junior drove his rig on Interstate 10 from San Antonio to El Paso. At the halfway point he noticed that he had been averaging 80mph, while his company requires his average speed to be 60mph. What must be his speed for the last half of the trip so his speed will average 60mph for the trip?

Answer
You might be tempted to say 40 mph but that would be incorrect.                                            average speed = total distance/total time
let 2x = total distance and s = speed in last half
time for first half = x/80
time for second half = x/s
so 60 = 2x/[x/80 +x/s]
60 = 2/[1/80 +1/s], s = 48 mph