Slices of the Whole

A fraction describes a part of something. Cut a pizza into 8 equal slices and eat 3 — you ate 3/8 of the pizza. The bottom number (denominator) tells you how many equal parts exist. The top number (numerator) tells you how many parts you are considering.

Fractions are not just math class notation. Every time you say "half an hour," "a quarter tank of gas," or "three-fifths of the vote," you are using fractions in plain language. They are the most natural way to describe portions and shares. Ancient Egyptians used only unit fractions — fractions with numerator 1 — expressing 2/5 as 1/3 + 1/15, a constraint that forced elegant decompositions mathematicians still study.