math
posted by val .
A pilot can fly an MD11 2160 miles with the wind in the same time she can fly 1920 miles against the wind. If the speed of the wind is 30 mph, find the speed of the plane in still air.( Source: Air Transport Association of America)

time = distance/speed
since the times are the same,
2160/(s+30) = 1920/(s30)
s = 510 mph