SOLUTION:
The formula for calculating the effective interest rate on a simple discount note is:

where:
- Discount is the interest charged on the loan.
- Face Value is the amount borrowed.
- Days is the length of the loan in days.
In this case, the Face Value is $26,900, the Ordinary Bank Discount Rate is 15%, and the Days is 30. We need to calculate the Discount first:



Now we can plug in the values into the formula for Effective Interest Rate:




The effective interest rate on the simple discount note is 18%.
