Speed is fast when there is no wind! The simplest method is the number of belts. Suppose the distance is 3000, the plane speed is 2, the wind speed is 1, the round-trip time is 3000 when there is no wind, and the round-trip time is 4000 when there is wind.

If it is not a decimal operation, let the distance be d, the plane speed be v, the wind speed be u, the windless time be 2d/v, and the windless time be (v+u) divided by d plus (v-u) divided by 2dv/(the square of v -u), and the numerator and denominator be divided by v at the same time. When there is no wind, we can see that the molecules are the same and the denominator is less than.