calcMinimumPayment is a method that accepts a double value for the current balance as a parameter and returns the calculated minimum payment due based on the customer's current balance.
The minimum payment is calculated according to following table:
| If the current balance is: | the minimum payment due is: |
| less than or equal to zero | 0 |
| greater than 0 but less than or equal to $25.00 | $25.00 |
| greater than $25.00 | $25.00 plus 2% of the current balance - $25.00 |