A phone credit balance follows B = 45 - 3.75d, where B is the balance in pounds and d is the number of days. On which day does the balance reach zero?

1. The balance reaches zero when B = 0, so write 0 = 45 - 3.75d. 2. Add 3.75d to both sides to get 3.75d = 45. 3. Divide both sides by 3.75 to get d = 12. 4. Check by putting d = 12 back in: 3.75 pounds times 12 days is 45 pounds, and 45 minus 45 is 0. 5. Option A comes from dividing 45 by 3, which rounds the daily charge down and stretches the credit too far. 6. Option B comes from dividing 45 by 2.5, another rounding of the same kind. 7. Option D comes from dividing 45 by 5, rounding the daily charge up instead. _Source: OpenStax College Algebra (CC BY 4.0), Ch 4 "Linear Functions", section 4.1 Linear Functions_