Let's define the next variables:
x: quarts of sparkling water
y: quarts of grape juice
We want to make 100 quarts of punch, then
x + y = 100
In order to follow the recipe we need to keep the next ratio:
Replacing this result into the first equation:
Replacing into the previous formula:
You would need 66 2/3 quarts of sparkling water and 33 1/3 quarts of grape juice to make 100 quarts of punch