Answer:
15/8 quarts
Step-by-step explanation:
The recipe calls for 1 1/2 quarts of sparkling water. This can be written as the improper fraction 3/2 (this is because 1 whole = 2/2; 2/2 + 1/2 = 3/2).
Using 15/4 quarts sparkling water means we used 5/2 times as much as the recipe calls for. This means we would use 5/2 times the amount of grape juice as well; 5/2(3/4) = 15/8.