Hyphen in Compound Numbers

Is it correct to use hyphens in numbers? Sometimes. When writing a compound number—any number made up of two words—we use a hyphen between each word. This applies to any number between twenty-one (21) and ninety-nine (99). Round numbers higher than ninety-nine don’t require a hyphen. Look at the following examples of correctly placed hyphens in numbers:

Fifty-six bottles of pop on the wall, fifty-six bottles of pop . . .
No, I won’t party “like it’s nineteen ninety-nine.”
Students generally finish their post-secondary education when they’re about twenty-four or twenty-five years old.

The hyphens-in-numbers rule applies even when the compound number is preceded by other numbers that do not require hyphens. See the examples below:

One hundred and thirty-three
Six thousand and seventy-two
One hundred million, forty-four thousand, nine hundred and eighty-five
We baked three hundred and twenty-five cookies for the bake-off.

It also still applies when the compound number (i.e., a number between 21 and 99) is used as an adjective. The examples below illustrate this usage:

Seventy-five billion dollars is not a lot of money when you’re researching space.
If twenty-two million people died, it would be called “a disaster”; if twenty-two million trees die, we call it “the pulp-and-paper industry.”

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