We are told that every 1 apple pie requires 6 cups of apples, so every time we make a pie, we add 6 cups of apples.
For 1 pie, we need 6 cups.
For 2 pies, we need 12 cups (6 + 6).
For 3 pies, we need 18 cups (12 + 6).
And for 4 pies, we need 24 cups (18 + 6).