A plane flies 720 km with the wind and 630 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?  Choices:
A) 205 km/h
B) 215 km/h
C) 225 km/h
D) 235 km/h
E) 245 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   720 / (x+15) = 630 / (x-15)   720(x-15) = 630(x+15)   72x - 1080 = 63x + 945   9x = 2025   x = 225   Therefore, the speed of the plane in still air is 225 km/h.   The answer is 225 km/h.
<result>C</result>
A plane flies 720 km with the wind and 630 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?