Just guess and check. For example, for the first on the left, I guessed 3 to begin with. 3 + 3 + 3 = 9, but 10 + 3 = 13, so 3 is not the answer. I then guessed 5. 5 + 5 + 5= 15, and 10 + 5= 15, so star = 5.
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