Problem 6.4. In a coastal village, 7 people go fishing every day, 8 people go fishing every other day, 3 people go fishing every three days, and the rest do not fish at all. Yesterday, 12 people went fishing, and today, 10 people are fishing. How many people will go fishing tomorrow?
Answer: 15.

Solution. Let's calculate how many times in total they fished yesterday and today. 7 people who fish every day fished 2 times each, i.e., a total of 14 times. 8 people who fish every other day fished exactly 1 time each, i.e., a total of 8 times. Therefore, these 15 people fished a total of $14+8=22$ times over the past two days.

According to the problem, they fished $12+10=22$ times in total over the past two days. Therefore, the fishermen who fish every three days were not present yesterday and today, and they will all come to fish tomorrow. Additionally, 7 people who fish every day will come tomorrow, and $12-7=5$ fishermen who fish every other day will also come. Thus, a total of $3+7+5=15$ people will come to fish tomorrow.