1. Petya's watch gains 5 minutes per hour, while Masha's watch loses 8 minutes per hour. At 12:00, they set their watches to the school clock (which is accurate) and agreed to go to the rink together at half past six. How long will Petya wait for Masha if each arrives at the rink exactly at 18:30 by their own watches?
Answer: 1.5 hours.

Solution: Petya's clock runs at a speed of 13/12 of the normal speed, so 6.5 hours on his clock pass in 6 hours of real time. Masha's clock runs at a speed of $13 / 15$ of the normal speed, so 6.5 hours on her clock pass in 7.5 hours of real time.