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

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Attn: Here I assumed w = 15 mph.