Ava planted 9 apple trees in her backyard. That's 3 more than Lily planted in her backyard. How many apple trees did they plant in their backyards collectively? Let's think first. Chain of thought:
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Lily planted 9 - 3 = 6 apple trees in her backyard. They planted 9 + 6 = 15 apple trees in their backyards collectively.
Therefore, the answer is 15.
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