Question 1189735
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I'm using the formulas found on this page
https://www.mtgprofessor.com/formulas.htm


The home value is $198,741.
20% of it is paid off through the downpayment, so 80% is remaining
0.80*198741 = 158,992.80


This is the loan amount L
L = 158,992.80


For the original mortgage at 11.4%, the monthly rate c is
c = r/12 = 0.114/12 = 0.0095
and the number of months is n = 360 because 30*12 = 360.


So,
L = 158,992.80
c = 0.0095
n = 360
Plug those values into the first formula mentioned in that link above. You should get a monthly payment of P = 1562.37


Plug those values into the second formula along with lowercase p = 120 (since 10 years = 120 months) to find that B = 147,456.55
Unfortunately that link uses uppercase P to represent payment and lowercase p to denote the number of months paid into; the letter choice may seem a bit confusing. 


Anyways, we get B = 147,456.55 which is the unpaid balance at year 10. This is the starting point, and we'll have two branching options


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starting balance = 147,456.55


Mortgage A (interest rate = 11.4%)
monthly payment = 1562.37
total amount paid back = (240 months)*(1562.37 per month) = 374,968.80
m = Total interest paid 
m = 374,968.80 - 147,456.55
m = 227,512.25


Mortgage B (interest rate = 6.6%)
monthly payment = 1108.09 
total amount paid back = (240 months)*(1108.09 per month) = 265,941.60
n = Total interest paid 
n = 265,941.60 - 147,456.55
n = 118,485.05


m-n = amount of interest saved by going with mortgage B
m-n = 227,512.25 - 118,485.05
m-n = <font color=red>109,027.20</font>



Answer: <font color=red>$109,027.20</font>
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