onefifth

One fifth, written as 1/5, is a fraction describing one part of a whole that is divided into five equal parts. If you partition a whole into five congruent pieces, one piece represents one fifth of the whole.

As a decimal, 1/5 equals 0.2, and as a percentage it equals 20%. It is a unit fraction since the numerator is 1. In base-10 arithmetic, the decimal terminates because the reduced denominator has only the prime factors 2 and 5.

One fifth is used in everyday measurement, recipes, and when dividing quantities into five equal parts. In probability, a fair five-outcome event assigns probability 1/5 to each outcome.

The term fifth is derived from the Old English fift, related to the word five; one fifth denotes a single part of five equal parts. The concept is closely related to the idea of a reciprocal: the reciprocal of five is 1/5, and 1/5 is the simplest form of the fraction representing one part of five.