10. There is an electronic clock that lights up every 9 minutes and rings at every whole hour. At 12:00 PM, the electronic clock both rings and lights up. When will it next both ring and light up?
3 o'clock
10.【Solution】Since the electronic clock rings at every hour, we only need to consider which hour the light will turn on. Starting from noon 12 o'clock, the light turns on every 9 minutes. How many 9-minute intervals will pass before it reaches an hour? Since 1 hour = 60 minutes, this question can be rephrased as: how many times 9 minutes is a multiple of 60 minutes? That is, find the least common multiple of 9 and 60. The least common multiple of 9 and 60 is 180. This means that 180 minutes after noon, which is 3 hours, the electronic clock will ring and light up again.
Answer: The next time it rings and lights up is at 3 o'clock in the afternoon.