Chapter 3.1, Problem 83E

In Problems $83\text{-}86$, assume that the annual interest rate on a credit card is $19.99\%$ and interest is calculated by the average daily balance method.

The unpaid balance at the start of a 30-day billing cycle was $\text{\$654}\text{.71}$. No purchases were made during the billing cycle and a payment of $\text{\$654}\text{.71}$ was credited to the account on day $21$. Find the unpaid balance at the end of the billing cycle.