Bunuel wrote:

Two men A and B walk from P to Q a distance of 21 km at rates 3 and 4 km an hour respectively. B reaches Q and returns immediately and meets A at R. The distance from P to R is:

A. 14 km

B. 16 km

C. 18 km

D. 20 km

E. 22 km

We can let t = the time, in hours, it takes B to meet A at R. We also must recognize that when they meet at R, together they have walked exactly twice the distance of 21 km (of course, B walks more than 21 km and A walks less than that). Therefore, we can create the equation:

3t + 4t = 2(21)

7t = 42

t = 6

Therefore, A and B both have walked 6 hours when they meet at R. Therefore, the distance from P to R is also the distance walked by A, which is 6 x 3 = 18 km.

Answer: C

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