A plan flies 420 miles with the wind and 350 miles against the wind in the same length of time . If the speed is 15mph, find the speed of the plane still in the air.
Let x be the speed of the plane in the still air, and w be the wind speed.
Balance by the time,
420/(x+w) = 350/(x-w)
Cross multiplication after canceling 70 gives,
6(x-w) = 5(x+w)
=> x = 11w = 11*15 = 165 mph <==Answer
Attn: Here I assumed w = 15 mph.