Alright, lets get started.
A light private plane can fly 120 mph in still air.
Suppose the speed of wind = x mph
Against the wind, speed of plane will be = 120-x
With the wind, speed of plane will be = 120+x
Flying against the wind, the plane can fly 320 miles, so time taken will be
t =
With the wind, the plane can fly 640 miles, so time taken will be
Both times are given same, so
Cross multiplying
Dividing from 320
x = 40 mph
So, the speed of the wind is 40 mph. : Answer
Hope it will help :)