13. Minutkin usually wound the clock to the limit twice a day: in the morning at 8:30 and at night when going to bed. In the morning, he had to make 9 full turns of the clock's crown, and at night - 11. At what time did Minutkin go to bed?
13. Over the night, the clock spring unwound by 9 turns of the head, during the day - by 11, and over 24 hours - a total of 20 turns $(9+11=20)$. One turn of the clock head allows the spring to work for 1.2 hours $(24: 20=1.2)$. Thus, from the time Minutkin went to bed until 8:30 AM, 10 hours and 48 minutes passed $(1.2 \cdot 9=10.8)$. Therefore, Minutkin went to bed at 9:42 PM (10 hours 48 minutes - 8 hours 30 minutes $=2$ hours 18 minutes, 12 hours - 2 hours 18 minutes $=$ $=9$ hours 42 minutes).