As can be seen from the meaning of the question, the round trip on a flat road is the same, that is to say, the time for going uphill is 4 minutes longer than that for going downhill, which is115 hours. If the length of the uphill road is x kilometers, there are

x/ 10- 1/ 15 = x/ 18

Solution, x= 1.5

Then the uphill time is1.5/10 = 0.15 hours =9 minutes.

So leveling the road takes 29-9=20 minutes = 1/3 hours.

The length of flat road is 15 * 1/3 = 5km.

So the distance from A to B is 1.5+5 = 6.5km.

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