Wanda takes out a 28-year mortgage for $192000 at an interest rate of j_1= 7.8%. After the 10th monthly mortgage payment, she decides to make some changes. To repay the loan, she will make 90 more mortgage payments (additional to the 10 she has already made). At the same time as the 11th payment (that is, one month from the 10th payment), she plans to set up a sinking-fund that will pay off the remainder of the mortgage on the same day as she makes the last mortgage payment. If the sinking-fund earns interest at j_12= 5.7%. What will be the amount of each monthly sinking-fund deposit?
Note: She will make her first deposit into the sinking-fund on the same day that she makes her 11th mortgage payment. This question was posted 4 years ago but I think the explanation given might be incorrect, so I'm asking it again.
First I find the monthly interest rate for $j_1=7.8%$ and I get $i_1=0.006276584325$. I find the monthly payments of the mortgage where:
$$R = 192,000/((1-(1+i_1)^{-336})/i_1)$$
$$R = $1,373.13$$
Then I find the outstanding balance after those 10 payments:
$$B_{10} = 1373.13*((1-(1+i_1)^{-326})/i_1)$$
$$B_{10} = $190,275.16$$
Then to find The Sinking Fund Deposit, I have the interest rate $j_{12} = 5.7% $ so $i_2 = 0.00475$
$$R = 190275.16/(((1+i_2)^{90}-1)/i_2)$$
$$R = $1699.31$$
However, my answer is incorrect. I'm not sure where my error is.