Echar ganas
Can I say for someone who studied a lot for an exam and succeeded it: "Echo muchas ganas." Meaning, she studied a lot, that's why she passed the exam.

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"Le echó muchas ganas" . That is the right way to say that.

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Exacto Jane, puedes decir "le echó muchas ganas, y por eso aprobó" Buen trabajo

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"Echarle ganas" is the expresión and it has meaning on its own. Your intuition is right.

No le eché ganas => I didn't try it
Le eché ganas => I tried it.
Le eché pocas ganas => I didn't try it hard enough
Le eché muchas ganas =>  I tried it really hard.
Le eché todas mis ganas => I tried it as hard as I could

Where I live we use "ponerle ganas" and "echarle ganas" in the same way.