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I got this pricing from Apple's website. It says $1449$ or $65.52$ per mo for 24 months at 7.99% APR.

Following the example from Mathematics of Investment and Credit (7th Ed), section 1.4 (Nominal interest rates) I got:

$\frac{7.99}{12} = 0.666$

And assuming compounding, we have $1449*(1 + 0.00666)^{24} = 1699.24$

But then I multiply $65.52 \times 24$ and get $1572.48

What is going on? My number $1699.24 seems right according to the example in the book I consulted.

Any assistance is much appreciated.

BeefStew
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1 Answers1

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The \$1699.24 number you calculated is the amount paid if you never pay anything against the principal of the loan until the very end. However, in each payment, you are reducing the principal of the loan, such that for each subsequent payment you are paying less interest. Note how this is a different situation to leaving money at a bank without taking any out, thus reducing the principal of your loan to the bank.

Where $p$ is the monthly payment, $i=0.666\%$ is the monthly interest rate, and $V_x$ is the amount owed after the $x^{\rm th}$ payment, this loan is described by the equation

$$V_{x+1}=(1+i)V_x-p$$

This sort of equation can be solved using z-transforms, and perhaps a few other ways, but for this application it suffices to confirm that the given value of $p$ indeed gives $V_{24}=0$.

Using a computer to evaluate the above equation to find $V_{24}$, we get within \$0.20 of zero, confirming that the given price per month is likely correct to within \$0.01.


To contrast, consider the equation where you make no monthly payments. We have

$$V_{x+1}=(1+i)V_x$$

which is easily reduced to

$$V_x=(1+i)^xV_0\rm.$$

You may recognize this formula as the one you used for compound interest.


Using the z-transform as mentioned above, the equation with monthly payments can be reduced to

$$V_x=V_0(1+i)^x-\frac{p}{i}\left((1+i)^x-1\right)$$

which can be solved for $p$ to find the monthly payment.

Angelica
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  • I guess this only works if every payment goes equally into interest and principal, like in this case. However, with many banks (especially the more disreputable ones) you actually pay the interest first and after that's gone, only then the payments go towards the principal, so I guess the formula will be a bit different in this case, right? In any case, +1 for the great answer :-) – Hans Olo Aug 25 '24 at 09:52
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    @HansOlo Monthly payments can be thought of as split into interest and principal, but that distinction doesn't actually exist in the math---each month, interest is added and payments are deducted. With a constant monthly payment, paying more interest at the beginning is simply a consequence of having a larger balance accruing interest---this is true of just about any loan. – Angelica Aug 25 '24 at 14:06
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    It's in the numbers where disreputable lenders come into play, allowing consumers to make interest-only payments or very long loan periods. For example, a 5-year loan would allow payments of $29.37, paying $313 instead of $123 in interest. – Angelica Aug 25 '24 at 14:08
  • Thanks for the clarification @Angelica, it's really appreciated :-) – Hans Olo Aug 25 '24 at 15:59