distance=speed times time. Therefore time=distance/speed.
The speed in these equations is the combined speed of the aircraft and the wind, which depends on the direction the aircraft is flying.
If the speed of the wind is w, we add the wind speed to the plane’s speed for downwind and subtract the wind speed for upwind:
200/(335+w)=135/(335-w), because the flight times are equal. Cross-multiplying: 200(335-w)=135(335+w).
67000-200w=45225+135w, 21775=335w, and w=21775/335=65mph.