If I were walking at 4 km / h I would arrive five minutes late to school. But as I will go at 5 km / h I will arrive ten minutes before the time of entry.
How far is the school from my house?
The school is 5 km away. If they were two brothers simultaneously ( TO Y B ) Those who go to the School, one walking at 4 km / h and the other at 5 km / h, the distance to school would be the one that can be traveled with a difference of 15 minutes between them.
So in the first kilometer TO, which walks at 4Km / h would take 15 minutes and B would take 12 minutes. At 2 km TO it would have taken 30 minutes and B 24 minutes so that if we continue the progression until the difference between the route of both is 15 minutes we see that the distance to the school has to be 5Km. TO it would take 75 minutes and B it would take 60. The difference, 15 minutes, is the one proposed by the statement.