hyphen in compound numbers and fractions
Twenty-one and two-thirds.
Hyphenate the compound numbers from twenty-one through ninety-nine: twenty-one, forty-two. Hyphenate a simple fraction in every use, as a noun or a modifier: two-thirds of the votes, a two-thirds share. See also fractions: agreement, hyphenation, and style.
Examples
Use twenty-one keys
Use a two-thirds share
Use two-thirds of the votes
Avoid twenty one keys
Avoid two thirds of the votes