Geni is a customer of a telephone company that offers the following plan:

- a fixed monthly fee of R$ 18.00;
- 10 hours of free calls per month;
- R$ 0.03 per minute exceeding the 10 free hours.

In January, Geni used his phone for 15 hours and 17 minutes and, in February, for 9 hours and 55 minutes. What was Geni's expense with the phone over these two months, in reais?
(a) 45.51
(b) 131.10
(c) 455.10
(d) 13.11
(e) 4.55
The correct option is (a).

Let's look at the expense in January. Since 10 hours are free and Geni used the phone for 15 hours and 17 minutes, she needs to pay the fixed monthly fee of 18 reais plus the cost of only 5 hours and 17 minutes. Since the price is given per minute, we convert the time to be paid into minutes. We know that 1 hour = 60 minutes, so 5 hours = 5 × 60 = 300 minutes. Therefore, 5 h 17 min = 300 + 17 = 317. Thus, Geni's phone bill in January was 18 + 317 × 0.03 = 18 + 9.51 = 27.51 reais.

In February, Geni used her phone for less than 10 hours, so in this month, she only needs to pay the fixed monthly fee of 18 reais. Therefore, Geni's total expense with the phone over these two months was 27.51 + 18 = 45.51 reais.