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
Usetwenty-one keys
Usea two-thirds share
Usetwo-thirds of the votes
Avoidtwenty one keys
Avoidtwo thirds of the votes