751.36 [(1+0.0399/12)^(12*30) - 1]/(0.0399/12)/(1+0,0399/12)^(12*30)

751.36 [(1+0.0399/12)^(12*30) - 1]/(0.0399/12)/(1+0,0399/12)^(12*30) =

751.36 x                                              209.71441.........

=$157,571.02 - This is the PV of a loan, mortgage, annuity....etc. with a monthly payment of $751.36 @ 3.99% over a period of 30 years.

Given the same monthly payment of $751.36 and the same interest rate of 3.99% over a period of 15 years or 180 months, the Present Value of the loan will be =$101,646.82.

Well, if your monthly payment cannot exceed 28% of $751.36, which means: $751.36 x 28% =$210.38, then the question becomes: What loan amount can you afford to borrow at 3.99% over a period of 15 years? The answer would be =$28,461.00.