Sienna faced unexpected travel costs of $⁢1,500 , charged to a credit card with a 15% APR. If a represents the average daily balance and b is the finance charge for one month, which equation accurately models b using the APR?

A 15% APR is the yearly interest rate, so the monthly rate is 0.15 ÷ 12, or 0.15/12. Using the average daily balance a for a 30-day month, the finance charge is b = (0.15/12) × a × 30. This matches choice C, b = (0.15/365) × a × 30; the daily rate is based on 365 days, not 12 months directly.

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