Final answer:
Juanita will be paying an effective annual interest rate of 9.99783% on her credit card balance.
Step-by-step explanation:
The effective annual rate of interest can be calculated using the formula:
EAR = (1 + nominal interest rate / n) ^ n - 1
where EAR is the effective annual rate and n is the number of compounding periods in a year.
In this case, the nominal interest rate is 9.9% and interest is billed monthly, so n = 12. Plugging in these values into the formula:
EAR = (1 + 9.9 / 100 / 12) ^ 12 - 1
= (1 + 0.00825)^12 - 1
= 0.0999783 or 9.99783%
Therefore, the effective annual rate of interest that Juanita will be paying on her credit card balance is 9.99783%.