Teacher Ana went to buy cheese bread to honor the students who won in the OBMEP, given that

- every 100 grams of cheese bread cost $R \$ 3.20$ and correspond to 10 cheese breads; and
- each person eats, on average, 5 cheese breads.

In addition to the teacher, 16 students, one monitor, and 5 parents will be present at the party. The precision of the bakery's scale is 100 grams.

(a) How many grams of cheese bread should she buy so that each person can eat at least five cheese breads?

(b) In this case, how much will the teacher spend?

(c) If each person eats five cheese breads, will there be any cheese bread left over?
(a) The teacher + 16 students + 1 supervisor + 5 parents $=23$ people will eat the cheese bread. To ensure that each person can eat at least 5 cheese breads, it is necessary to buy, at a minimum, $5 \times 23=115$ cheese breads. On average, each cheese bread weighs $\frac{100}{10}=10$ grams, so it will be necessary to buy

$$
10 \times 115=1150 \text{ grams of cheese bread.}
$$

Since the scale's precision is $100 \text{ g}$, we round $1150 \text{ g}$ to $1200 \text{ g}$ and obtain the amount of cheese bread the teacher should buy, in grams.

(b) Since $\frac{1200}{100}=12$, the teacher will spend $12 \times 3.20=38.40$ reais.

(c) The amount of cheese bread bought was $\frac{1200}{10}=120$ pieces. Therefore, $120-115=5$ cheese breads will be left over.