While Burak only pays the interest of the 20,000 TL debt he has taken to repay in 15 years, he also creates a debt payment fund that gains 9% annual value in order to be able to pay the principal at the end of the maturity period. However, from the 7th year, the rate of return of the debt repayment fund increases to 12%. Calculate the decrease in the installment amount paid by Burak to the debt repayment fund due to the increase in interest.

Please answer with finance mathematics formulas.