This is the problem of equal annuity in actuarial science. As a high school student, you don't know economics, so naturally you don't understand this discount problem. Let me put it in plain English.

First of all, the meaning of interest rate is the time value of money, that is, the value of 100 yuan today is equal to 100*( 1+r) yuan a month later. Similarly, the value of 100 yuan after one month is equal to 100/( 1+r) yuan today. Converting future money at the current interest rate is a discount. The basic principle of equal repayment is that the sum of the amount of money to be repaid in each installment in the future is equal to the principal A, so if the repayment is made at the end of the period, the X yuan for the first installment is now x/( 1+r), the money for the second installment is now X/( 1+R) 2, and the X yuan for the n installment is now X/(1+r).

Your second question is reversed. It is a truth that every repayment in the future is counted as the value of the last installment. In short, it is easy to understand that interest rate is not profit, and that one yuan today is not equal to one yuan tomorrow. I studied economics after I went to college, and I thought it was naive to do actuarial things.