5.1. Alina travels to school by bus. The bus runs on a schedule every 15 minutes. It always takes the girl the same number of minutes to get to the bus stop. If she leaves home at 8:20, she will be at school at 8:57, but if she leaves home at 8:21, she will be late for school. Classes start at 9:00. How many minutes will Alina be late for school if she leaves home at 8:23?
Answer: 12

Solution. Since Alina is late for school if she leaves home one minute after 8:20, leaving 1, 2, ... 15 minutes after 8:20, she will catch the next bus, which means she will arrive at school 15 minutes later than 8:57, i.e., she will arrive at school at 9:12.