Knowing that there are 4 quarts in one gallon, you can use this information to set up a proportion to find how many quarts are in 3 gallons. When you set up this proportion, you should get this where x is the amount of quarts:
4/1=x/3
Then, to solve for x, or your number of gallons, cross multiply:
1*x=4*3
When simplified, you should get that x=12, or that there are 12 quarts in 3 gallons :)
