A plane flies 400 miles with the wind and 320 miles against

A plane flies 400 miles with the wind and 320 miles against the wind in the same length of time. If the speed of the wind is 20 mph, whqat is the speed of the plane in still air?

Given the speed of wind is 20mph.
let the speed of plane be x
Time taken is same for both sides.
so equating time taken we get ,
400/x+20 =320/x-20
solving this we get wind speed as 180 mph.