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I thought '30-day' and '30 days' are right. But, when I typed 30 days plank challenge, all subject was written as 30 day. Can I use 30 day instead of 30 days? It's tricky! Please help me.

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When you are using it as a noun phrase, you say "30 days". When you are using it as an adjective, you say "30-day".

For example, "The project will last for 30 days", versus "This is a 30-day project".

Similarly for other number-plus-unit expressions.

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