A wall clock is seen in a plane mirror and the mirror image shows the time as 8:20. What is the actual time on the clock?

For a mirror image of a clock face, subtract the shown time from 12:00 (use 11:60 to make the subtraction easy). 11 hours 60 minutes − 8 hours 20 minutes = 3 hours 40 minutes. So the actual time is 3:40.