Answer:
$557.51
Explanation:
A financial calculator tells you the payments are ...
on $80,000 at 4.75%: $417.32
on $20,000 at 7.525%: $140.19
Then the total monthly payment is ...
$417.32 +140.19 = $557.51
_____
You can use the amortization formula to find the payment (A) on principal P at interest rate r for t years to be ...
A = P(r/12)/(1 -(1+r/12)^(-12t))
I find it takes fewer keystrokes to enter the numbers into a financial calculator. Both give the same result.