At that rate, about how many gallons of milk will they need to purchase in a year’s time? Give your answer as a whole number.

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.