Math: How to calculate APR

Hi guys, are there any mathematicians here who can help me,
ive been trying to work out how to work out how finance is calculated (that was a mouthful huh).
For example, using http://www.midlandcredit.co.uk/car-loans/car-finance-calculator/
Price of vehicle: 20000
Deposit: 2500
Pay over: 60 months
Credit: Excellent APR: 5.99%
This site recons, total repayment is �20120.63, credit being �2620.63, monthly payments of �335.34
I tried working it out like this:
Loan (20000) – Deposit (2500) = �17500
Divide the interest rate (5.99s by 100 to get it as a decimal = 0.0599
Work out how many years 60 months is (5 years).
Then do: �17500 x 0.0599 x 5 = �5241.25
Which is the cost of the credit (the interest), add this to the loan, �17500 + �5241.25 = �22741.25
Where have I gone wrong?
Thanks in advance,
Answer #1
You’re calculating the interest wrong. You’re balance, the amount you’re paying interest on, is reduced every month that you make a payment.
It also depends on how the lender calculates interest on the loan.
read more here…
http://www.wikihow.com/Calculate-Annual-Percentage-Rate