A cyclist drove one kilometer, with the wind in his back, in three minutes and drove the same way back, against the wind in four minutes.If we assume that the cyclist always puts constant force on the pedals, how much time would it take him to drive one kilometer without wind?

Let c be the cyclist's speed on still air and w the wind speed (both in km/min). With the wind: c+w = 1/3; against the wind: c-w = 1/4. Adding these gives 2c = 1/3+1/4 = 7/12, so c = 7/24 km/min, meaning the still-air speed itself is 7/24 km per minute. The time for 1 km without wind is the reciprocal, 24/7 minutes, which is why that fraction is the correct choice.

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