Some bottles of juice are on sale with “buy 3 get 1 free”, and each juice costs $3. Danna wants to buy 12 such bottles. How much does she need to spend?

$

3 + 1 = 4, so take 4 bottles of juice as one group.

3 × 3 = $9, each group of juice costs 9 dollars.

12 ÷ 4 = 3. There are 3 groups of juice.

Thus, she should pay: 9 × 3 = $27