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At first, Mandy has 3 apples. So if she gives 1 to her friend, then she'll have ⇨ 3 - 1 = 2 apples left. Now, if she decides to buy 2 more apples, then she'll have ⇨ 2 + 2 = 4 apples. So, Mandy has 4 apples.
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Hope it helps.
RainbowSalt2222