Francis has a credit card that uses the previous balance method. The opening
balance of one of his 30-day billing cycles was $0, but this was his balance
for only the first 15 days of the billing cycle. He then made a purchase that
increased his balance to $3600, and his balance stayed this amount for the
remainder of the billing cycle. If his credit card's APR is 21%, how much was
Francis charged in interest for the billing cycle?