Spell out fractions standing alone or followed by of a, of an, or of the.
   The gap was three fourths of an inch.

Hyphenate fractions when they are used as adjectives and spelled out.
   There was a two-thirds majority in favor.

Use numerals for mixed fractions unless they begin a sentence.
   The recipe calls for  3 1/2 tablespoons of oil.
   Four and one-quarter percent is the current interest rate.