Math problem in the fourth grade of primary school: The teacher prepared 10 for two students, and now there are three more students. The daily expenses of each student remain unchanged. How many days can it last? Think of it this way: If no one spends one yuan a day, then the cost of 12 students and 10 days is 120 yuan. Now 15 students spend one yuan a day, so the number of days is 120/ 15 = 8. I remember my teacher in the fourth grade.