Reading a clock in the mirror

Question

You glance at an analog clock in a mirror. The reflection appears to read 2:15. What is the actual time? And what is a fast rule for converting any mirror reading to the real time?

Accepted Answer

The real time is 9:45. Why reflection flips the clock. A vertical mirror reverses left and right, so the whole clock face is mirrored. A hand pointing to a position at angle 	heta (clockwise from 12) appears in the mirror pointing to angle -	heta, the reflection of "20 past" looks like "20 to," and so on. The 12 and 6 stay put (they're on the mirror's axis), but everything else swaps sides. The quick rule. For a mirror reading of H{:}M, the actual time is 11	ext{:}60 \ - \ H	ext{:}M, that is, subtract the reading from 11:60 (which is really 12:00, but writing it as 11:60 makes the minute …

Reading a clock in the mirror

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