[TiK]

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"an investors' consent"

He insists it is correct. It means one investor, out of a group of investors, is giving his consent...Could one of you please explain why this phrase is correct?

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It's incorrect. If he was describing many investors and needing their consent, then it'd be "the investors' consent," but since that sounds so clunky, I'd probably say the consent of the investors. But if he wants it to mean the consent of one investor, the correct way would be "the/an investor's consent."