A. The distance between B and a is 200km. A and B set out from a and B at 60km / h and 40km / h respectively If two people walk in the same direction along the AB direction, B is in front of a and B is behind a. a starts for 50 minutes, B starts again, and a few hours later catch up with B?

50 points = 5 / 6
（200-5/6×60）÷（60-40）
=150÷20
=7.5 hours
A: first 50 minutes, then B. after 7.5 hours

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