Answer: 18 quarts
Step-by-step explanation:
Quantity of sparkling water and gape juice are used in proportion in the recipe.
Let x= quantity of sparkling water when 9 quarts of grape juice is used.
Then,

Hence, 18 quarts of sparkling water is required to mix with 9 quarts of grape juice.
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$64 is 80% of $80. To find how many percent of $80 is $64. $64/$80 = 0.8 or 80%
To find how many percent of decrease, just subtract 80-64=16; $16/$80= 0.2 or 20%