The difference between the real time and the time of the
mirror image is two hours and ten minutes (two and a half hours, minus the twenty
minutes of cycling). Therefore, the original time on the clock at home that
morning could only have been five minutes past seven: The difference between these clocks is exactly 2 hours and ten minutes (note that also five minutes past one can be mirrored in a similar way, but this is not in the morning!).
Conclusion: The boy reaches school at five minutes past
seven plus twenty minutes of cycling, which is twenty-five minutes past seven!...