Kolya Vasin was walking after school for five hours. First, he walked along a horizontal road, then climbed a hill, and finally returned to the starting point via the old route. His speed was 4 km/h on the horizontal section of the road, 3 km/h while climbing the hill, and 6 km/h while descending the hill. What distance did Kolya Vasin cover?
For each section of the mountain road on the way there, Kolya spent twice as much time as on the way back.

Therefore, his average speed on the "mountain" road was $(2 \cdot 3 + 1 \cdot 6) : 3 = 4$ km/h, which is equal to the speed on the flat road. Consequently, he covered $4 \cdot 5 = 20$ km.

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