Using it's formula, it is found that the monthly payment should be of $688.49 in order to pay off the debt in 15 years.
It is given by:
[tex]A = P\frac{\frac{r}{12}\left(1 + \frac{r}{12}\right)^n}{\left(1 + \frac{r}{12}\right)^n - 1}[/tex]
In which:
For this problem, the parameters are given as follows:
A = 90000, r = 0.045, n = 15 x 12 = 180.
Hence:
r/12 = 0.045/12 = 0.00375.
Then we solve for A to find the monthly payment, as follows.
[tex]A = P\frac{\frac{r}{12}\left(1 + \frac{r}{12}\right)^n}{\left(1 + \frac{r}{12}\right)^n - 1}[/tex]
[tex]A = 90000(0.00375)\frac{(1.00375)^{180}}{(1.00375)^{180} - 1}[/tex]
A = $688.49
More can be learned about the monthly payment formula at https://brainly.com/question/22846480
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