You apply for a loan from the bank, and you want to borrow it for 30 years, and you want to persuade your parents to borrow it for 30 years. Then you can tell your parents that you can borrow it for 30 years. Now the monthly payment is very small, so the economic pressure is relatively small, and it is easy to repay the loan in 30 years. Therefore, parents are advised to apply for a 30-year period.