A plane flies 420 miles with the wind and 350 miles against the wind in the same length of time. If the speed of the wind is 23 mph, what is the speed of the plain in still air?
Choices:  A) 253 mph.  B) 378 mph.  C) 565 mph.  D) 431 mph.  E) 642 mph.
The speed of the plane in still air = x miles/hour   The speed of the wind is 23 mph   Speed with the wind = (x + 23) mph   Speed against the wind = (x – 23) mph   Time = Distance/ Speed   According to the problem,   420/(x + 23) = 350/(x – 23)   420(x – 23) = 350(x + 23)   420x – 9660 = 350x + 805   420x – 350x = 8050 + 9660   70x = 17710   x = 17710/70= 
<gadget id="calculator">17710/70</gadget>
<output>253</output>
253   Therefore, the speed of the plane in still air= 253 mph.   correct answer 253 mph.
<result>A</result>
A plane flies 420 miles with the wind and 350 miles against the wind in the same length of time. If the speed of the wind is 23 mph, what is the speed of the plain in still air?