be barking up the wrong tree

”be barking up the wrong tree”
To make a wrong choice or ask the wrong person; to be wrong about the reason for something or the way to achieve something
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Example 1: She thinks it’ll solve the problem, but I reckon she’s barking up the wrong tree.
Example 2: Our teacher said, “You’re barking up the wrong tree if you think I’m going to give you the answer to that test question.”