Discussion

Explanation:

Since the ratio of speeds of faster person to slower person is less than 2,

Time taken to meet nth time = $\frac{\left(2n-1\right)D}{R_s}$

where, 
D = distance between the end points.
Rs = Relative speed of the people running.

Here, time taken to meet for the third time = $\frac{\left(2\times 3-1\right)\times 100}{10+15}=20$hours.

Hence, 20.

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