Answer: 260 gallons
Step-by-step explanation:
We can use a proportion to solve this problem. We know that a family used 15 gallons of milk every 3 weeks. Since the problem asks for how much they will need for a year, we can gather that a year is 52 weeks. We can come up with the following proportion.
[cross multiply]
[divide both sides by 3]
Therefore, they will need to purchare 260 gallons.