A plane flies 660 km with the wind and 570 km against the wind in the same length of time. If the speed of the wind is 15 km/h, what is the speed of the plane in still air? Choose one:
A) 185 km/h B) 195 km/h C) 205 km/h D) 215 km/h E) 225 km/h
The speed of the plane in still air = x km/h   The speed of the wind is 15 km/h   Speed with the wind = (x + 15) km/h   Speed against the wind = (x â€“ 15) km/h   Time = Distance/ Speed   660 / (x+15) = 570 / (x-15)   660(x-15) = 570(x+15)   66x - 990 = 57x + 855   9x = 1845   x = 205   Therefore, the speed of the plane in still air is 205 km/h.   The answer is 205 km/h.
<result>C</result>
A plane flies 660 km with the wind and 570 km against the wind in the same length of time. If the speed of the wind is 15 km/h, what is the speed of the plane in still air?