Hyphens are used in the following instances:

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With numbers over twenty:

We ate twenty-two pizzas.

She owned forty-seven cats.

I only had ninety-four cents in my pocket.

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With compound nouns:

His sister-in-law was staying with us.

Their child is a four-year-old.

That coffee was a good pick-me-up.

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With compound adjectives:

The check-in time is not until this afternoon.

That was a once-in-a-lifetime vacation!

Their house had a two-car garage.