Initially, Charlotte owes $7680. She finishes her payments after a total of 6 + 36 = 42 months. Using a simple compounding formula, the amount she owes is worth P at the end of 42 months, where P is: P = 7680 * (1 + .2045/12)^42 = 15616.67379 Now, the first installment she pays (at the end of six months) is paid 35 months in advance of the end, so it is worth x * (1 + .2375/12)^35 at the end of her loan period. Similarly, the second installment is worth x * (1 + .2375/12)^34 at the end of the loan period. Continuing, this way, the last installment is worth exactly x at the end of the loan period. So, the total amount she paid equals: x [(1 + .2375/12)^35 + (1 + .2375/12)^34 + ... + (1 + .2375/12)^0] To calculate this, assume that 1+.2045/12 = a. Then the amount Charlotte pays is: x (a^35 + a^34 + ... + a^0) = x (a^36 - 1)/(a - 1) Clearly, this value must equal P, so we have: x (a^36 - 1)/(a - 1) = P = 15616.67379 Substituting, a = 1 + .2045/12 and solving, we get x = 317.82