Question

A plane traveling from Phoenix to Washington, D.C with a tailwind of 20 miles per hour takes 3 hours. The return trip in the same wind, which is now a headwind, takes the same plane 3 hours and 12 minutes. What is the average speed of the plane in calm conditions?
600 miles/hour
620 miles/hour
650 miles/hour
700 miles/hour

Answer 1

620 miles/hour

Answer 2

3(s+20) = 3.20(s-20)
s = 620
3*640 = 1920
3840/(3 + 3.20) = 619.35

so you are correct