Thirty-one

I was in fact thirty-one at the time.

She is thirty-one.

December has thirty-one days.

I've written down all the numbers up to thirty-one.

Tom is thirty-one.

In one year there are more than thirty-one million seconds.

Twenty-eight, twenty-nine, thirty, or thirty-one days make a month.

Twenty-eight, twenty-nine, thirty or thirty-one days make up a month.

Sara has thirty-one ballpoint pens.

A month has thirty or thirty-one days. Except February.