First,

18 months is 1.5 years, not 2.5.

Similarly, in "($9360 *20%*1/18)/100", I think you must have meant

1/12 (monthly) rather than 1/18. And if you do that, you are doing essentially the same thing as multiplying by 18/12 = 1/5.

The first method uses annual interest rate; the second uses the equivalent monthly interest rate. That is why both work.

And I find the problem ambiguous, particularly in the phrase "20% on the outstanding balance". Denis took this as it would usually be taken in practice: 20% (annual interest rate) on the outstanding balance after each payment, which reduces. This amounts to compound interest. Since they explicitly say it is simple interest, and you evidently are not working at that level, it seems that what they

mean is to precompute the simple interest on the balance after the deposit, and divide that into equal amounts. Which means that what you did, once you use the right numbers, is correct.

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