Each morning Boris walks to school. At one-fourth of the way he passes the machine and tractor station; at one-third of the way, the railroad station. At the machine and tractor station its clock shows 7:30, and at the railroad station its clock shows 7:35.(Source: The Moscow Puzzles: 359 Mathematical Recreations by Boris A. Kodermsky, edited by Martin Gardner)
When does Boris leave his house, when does he reach school?
An unstated assumption here is that walking speed is constant. If so, then five minutes is how long it takes to go $\frac{1}{3} - \frac{1}{4}$, or $\frac{1}{12}$ of the way. Accordingly it takes an hour for his total journey. But when does he start? By the time he reaches the machine and tractor station, it is 7:30. One-fourth of his journey, or fifteen minutes, have elapsed at this point, so he starts at 7:15 and gets there at 8:15.
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