A farmer has two types of milk, one that is 20% milkfat and one that is 12% milkfat. How much of each is needed to get 20 gallons of 15% milkfat?

8 gallons of 20% milkfat and 12 gallons of 12% milkfat

6.5 gallons of 20% milkfat and 13.5 gallons of 12% milkfat

7.5 gallons of 20% milkfat and 12.5 gallons of 12% milkfat

10 gallons of 20% milkfat and 10 gallons of 12% milkfat