Davi goes to a warehouse that sells a bottle of orange juice for $R \$ 2.80$ and a box with six of these bottles for $R \$ 15.00$. He needs to buy 22 bottles for his birthday. What is the minimum amount he will spend?
If Davi buys 6 bottles individually, he will spend

$$
6 \times 2.80=16.80 \text { reais }
$$

which is more expensive than buying a box of six. Therefore, he should buy the largest possible number of boxes. In this case, he should buy 3 boxes and 4 bottles individually, in which case he will spend

$$
3 \times 15+4 \times 2.80=56.20 \text { reais }
$$

which is the minimum possible.