7.4. Arseny sat down at the computer between 4 and 5 PM, when the hour and minute hands were pointing in opposite directions, and got up from it on the same day between 10 and 11 PM, when the hands coincided. How long did Arseny sit at the computer?
Solution: Let's see where the hands will be 6 hours after Arseny sat down at the computer. The minute hand will go around the clock 6 times and return to its place. The hour hand will move exactly half a circle. Therefore, the angle between the hands will change by 180 degrees, i.e., the hands will coincide. It is obvious that there is exactly one moment between 22 and 23 hours when the hands coincide, so this will be the time when Arseny gets up from the computer. Thus, Arseny sat at the computer for 6 hours.

## Criteria:

Only the answer - 1 point.

If in a similar solution it is not noted that such a moment between 22 and 23 is exactly one, deduct 1 point.