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You did NOT read this: http:#36573014

The 18 is the multiplier.

If your interests was for 20 days and you assume the year has 360 days then is the factor for 20 days this:

360 / 20 :== 18

The number of days to be 360 is a German bank calculation method to assume 12 months by 30 days.

But I think you see the question far too complicated.

All APR calculators try to solve the decrease of loan amount by monthly payment and increase of interest rate because you pay same amount every month for the interest.

But in your question you do not have monthly payments.

You confuse it because you have amount of days but you still pay back at once. So the APR is simply that percentage for repayment multiplied by the factor. In your example the factor is: 18

You see the 42 in the title of my example? That number is my favorite answer :-) Enter that number into Google and see what you get: 42 is the answer to all and everything if you know the question ;-)