Then he noticed the woman, who'd helped him earlier, gently holding a baby in her arms, both shaking.
(A sentence from my workbook)
Does this sentence work well? What (or who) does "both" refer to: arms, the woman, baby, or writer?
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You can't decide this grammatically, you can only decide this pragmatically. That is you need to use your common knowledge and common sense.
You are qualified to do this - it doesn't require any special skills in English. What makes more sense in the context of the whole story? Which would you find unsurprising? Which is rather ridiculous?
The woman and the baby were both shaking.
The woman's left and right arms were both shaking.