mkn: Suppose you purchase a car for a total price of $31,370
P = L[c(1 + c)^n]/[(1 + c)^n - 1]
n: number of months: 12*7
c: monthy interest rate: 9%/12=0.09/12
L: Loan amount
[input]31370*( 0.09/12*(1+0.09/12)^(7*12) )/((1+0.09/12)^(7*12)-1)[/input]
about 505$
[input]31370-505*7*12[/input]
that's 11050$ in interest!
+3144 
