Note (1) that the formula gives the same nominal annual interest rate as was calculated earlier, (2) that the first term is the periodic cost of the credit (SKI spends $1 to get the use of $99), and (3) that the second term is the number of “savings periods” per year (SKI delays payment for 40 - 10 = 30 days), and there are 365/30 = 12.1667 30-day periods in a year. Therefore, we could calculate the exact effective annual interest rate as: effective rate = (1.0101)12.1667 - 1 = 13.01%.
If SKI can obtain financing from its bank (or from other sources) at an interest rate of less than 13.01 percent, it should borrow the funds and take discounts