We always hyphenate the numbers 21 to 99 when writing them out as words.

For examples:

I have twenty-one pairs of novelty socks.

My grandma is sixty-seven years old.

We always hyphenate when writing an ordinal version of one of these numbers.

For example:

My forty-first birthday was the best day ever.

We always hyphenate when writing a larger number that contains one of these numbers.

For examples:

I ate one hundred and twenty-two burgers in 2019.

Around twenty-four million people eat fast food every day.

It’s only 21 to 99 that we hyphenate in these large numbers. Larger round numbers, such as “one hundred,” do not require a hyphen.

We always hyphenate adjectives that include a number when they appear in front of the noun they’re modifying. This applies to both words and numerals.

For examples:

We study seventeenth-century manuscripts.

I will send a 40-page document later today.

The 7-year-old boy lost his whistle.

However, these numbers don’t need a hyphen when they occur elsewhere.

For examples:

The ladder is six feet tall.

The document comprises 40 pages.

We hyphenate fractions when they’re used adjectivally in front of a noun, but not in other cases.

For examples:

A two-third majority voted for the new laws.

He ate two thirds of the cake.

Correct or incorrect?

Stella had her hair cut six-inches last week.

Correct or incorrect?

Her party shoes had three inch heels.

Correct or incorrect?

Can you lend me your five-foot tape measure?

Correct or incorrect?

The water level rose 10-inches in just three hours.