A recipe for sparkling grape juice calls for quarts of sparkling water and quart of grape juice. How much sparkling water would you need to mix with 9 quarts of grape juice?

For this case we can make the following rule of three:

1 1/2 > 3/4

x > 9

We clear the value of x:

x = ((9) / (3/4)) * (1 1/2)

Rewriting we have:

x = ((9) * (4/3)) * (1 1/2)

x = (36/3) * (1 1/2)

x = (12) * (1 1/2)

x = (12) * (3/2)

x = 18

you would need 18 quarts of sparkling water to mix with 9 quarts of grape juice