Rachel picked 4 apples from her tree. If the tree had 7 apples initially. How many apples are still there on the tree?
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Rachel picked 4 apples from her tree. 

That means that there are now 4 apples less on the tree. 

So if there were 7 apples to start with, and 4 apples were picked, that means that there are now 3 apples left on the tree.