Rhea had an unexpected medical emergency and had to pay $5,000 on her credit card. The card has an APR of 18% If x represents the average daily balance and y represents the finance charge for a month, which equation models the finance charge y accrued on the balance using the APR?

Convert the 18% APR to a daily rate by dividing by 365, then multiply by the average daily balance x and the 30 days in the month. This gives y = (0.18/365) · x · 30, which matches choice C. Choice D uses the full annual rate for a single month.

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