How long will a person sleep if he, going to bed at 9 pm, has set a mechanical alarm clock at 10 am?

We find out how many hours a person will be in a state of sleep on the day he went to bed, when we know by the terms of the assignment that this event took place at exactly nine o’clock in the evening:

9 pm = 21 hours 24 hours;

24 – 21 = 3.

We find out how many hours a person will be in a state of sleep when we know by the terms of the assignment that he should be woken up by an alarm clock at exactly ten o’clock in the morning of the next day:

10 + 3 = 13.

Answer: A person will stay in a state of sleep for thirteen hours.