A small vest worn by a monkey can be used as 10 yuan. Yesterday, Monkey Flower 10 yuan bought a small vest. This 10 yuan belongs to a bear, not a monkey. Monkeys can't keep 10 yuan as their own, and Pony Jia is also their own.

If you count this, everyone is a billionaire.

The method of mathematics is:

Yesterday, the monkey bought a small vest, which should be 10 yuan.

You should earn 10 yuan for returning the vest today.

You have to pay 15 yuan to buy another big vest.

15- 10=5, which should be paid to 5 yuan today.