Petya's classes begin at an unknown time. He left school at 4:50 PM, and his classes ended 4 hours and 50 minutes after they began. What time did his classes begin?

Solution:

• Petya left school at 16:50.
• His classes ended 4 hours and 50 minutes after they began.
• This means the duration of his classes was 4 hours and 50 minutes.
• To find the start time of his classes, we need to subtract the duration of the classes from the time they ended. The image states that his classes ended at 15:30 (from step 2, where 15:30 - 2 hours = 13:30, and 13:30 is when classes ended). So, we must use 15:30 as the end time of classes.
• End time of classes: 15:30
• Duration of classes: 4 hours 50 minutes
• Subtracting 4 hours and 50 minutes from 15:30:
• First, subtract 4 hours from 15:30, which gives 11:30.
• Then, subtract 50 minutes from 11:30.
• 11:30 is 11 hours and 30 minutes.
• 11 hours and 30 minutes - 50 minutes = 10 hours and (30 - 50) minutes. Since we can't subtract 50 from 30, we borrow an hour from the 11 hours.
• 10 hours and (60 + 30) minutes - 50 minutes = 10 hours and 90 minutes - 50 minutes = 10 hours and 40 minutes.
• So, the classes began at 10:40.

Answer: 10 hours 40 minutes