FMChpt 5 The mgt of receivables – Simple settlement discount

Dear John, one example you have given in the video:- A customer currently takes 25 days to pay. We then offer a discount of 1 % for payment within 10 days For every hundred we invoice, we would receive a hundred within 25 days, with the discount we will get it in 10 days. And because of the 1% discount we can only get 99 dollars. So for every 99 we get earlier, we are losing a dollar. The interest is therefore 1/99 = 0.010101 or 1.0101% over 15 days. Up to this point, I don't understand why the interest is calculated by 1/99. I think it is a mathematics problem. Is there any explanation to this? Thank you!