We call it and the space in kilometers spanning the hill, and t the time in seconds it takes the pilgrim to descend. As it rises 3 times slower, it will take 3t seconds to rise. So in total it takes 4t seconds to go up and down.
The average speed is the total space traveled (2e kilometers) divided by time (4t seconds), and taking into account that the pilgrim descends to 6 Km / h we have to:
V = 2e / 4t = 0.5. e / t = 0.5. 6 = 3 Km / h.
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