Answer: She require 25 cups of apple juice and 10 cups of sparkling water to make sparkling apple juice.
Explanation:
Given : Elena mixes 5 cups of apple juice with 2 cups of sparkling water to make sparkling apple juice.
i.e. the required ratio of apple juice to sparkling water = 5:2
The proportion of these ingredients remains the same .
In order to make 35 cups of sparkling apple juice, let we require 5x cups of apple juice and 2x cups of sparkling water to make sparkling apple juice.
Such that

i.e. She require 5(5) cups of apple juice and 2(5) cups of sparkling water to make sparkling apple juice.
i.e. She require 25 cups of apple juice and 10 cups of sparkling water to make sparkling apple juice.