# Commercial Mathematics. A question on payment. Need help.

Charlie buys a car for \$120,000. He pays half of the amount in cash and agrees to pay the balance in 12 annual instalments of \$5000 each. If the rate of interest is 12% and he pays with the instalment the interest due on the unpaid ammount, find the total cost of the car.

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total cost of the watch? you mean car? Also, how is the interest compounded? –  orlandpm Dec 15 '12 at 7:19
You say the cost of the car is $\$120,00$but you probably meant$\$120,000$ –  Henry Dec 15 '12 at 7:28

You just need a table of the thirteen payments, which might start something like

Year  Outstanding Interest Instalment Remaining
0     120,000        -     60,000    60,000
1      60,000     7,200     5,000    55,000
2      55,000     6,600     5,000    50,000


where

• the amount outstanding is equal to the amount remaining the previous year,
• the interest is 12% of the outstanding,
• the instalment is what you specified, and
• the remaining is the the amount outstanding less the instalment.

Then just add up the interest and the instalments.

You would get the same answer with $$120000+\sum_{k=1}^{12} k \times 5000 \times 0.12$$

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