First we need to solve the finance charge without the changes.
B = A (1 + ti)
where
t is the number of years, 60 months or 5years
i is the interest rate per year, 18% or 0.18
A is the principal amount, %15,600
B is the amount after t years
so,
B =$15,600 ( 1 + 0.18 x 5 )
B = $15,600 ( 1 + 0.9 )
B= $15,600 (1.9)
B= $29,640.
The finance charge without the changes is $29,640(amount after 5 years) - $15,600(principal amount) = $14,040.
Applying the changes, the principal amount would be $15,600 - $8,500 = $7,100, the number of years will be 2 years (24 months)in equation
B = $7,100 (1 + 0.18 x 2)
B = $7,100 (1 + 0.36)
B = $7,100 (1.36)
B = $9,656, the finance charge with the changes is $9,656 - $7,100 = $2,556.
The changes Jerry made can save $14,040 - $2,556 = $11,484.
