A boy leaves home in the morning to go to school. At the moment he leaves the house he looks at the clock in the mirror. The clock has no number indication and for this reason the boy makes a mistake in interpreting the time (mirror-image). Just assuming the clock must be out of order, the boy cycles to school, where he arrives after twenty minutes. At that moment the clock at school shows a time that is two and a half hours later than the time that the boy saw on the clock at home.
At what time did he reach school?

(In reply to there are 2 valid answers (or even 4) by Ady TZIDON)

The fact that, originally, both of us derived equations that only led to one of the two solutions suggests that we were making assumtions about modular arithmetic that are not quite quite correct. I'm still not sure why they were not, or how they differed so as to lead to the different solutions.

Posted by TomM on 2004-02-07 18:49:37