Chance as a Fraction

A bag holds 5 marbles: 2 green and 3 yellow. You reach in without looking. The probability of picking green is the number of green marbles over the number of all marbles: 2/5. Every marble is equally likely to come out, and 2 of those 5 equally likely picks are green.

In general, probability is the number of outcomes you want over the number of all outcomes. A die has 6 faces, and 3 of them are even (2, 4 and 6), so the probability of rolling an even number is 3/6, which is 1/2.

A probability always lies from 0 to 1. A probability of 0 means impossible, like rolling a 7 on a die. A probability of 1 means certain, like rolling a number from 1 to 6. The closer a chance is to 1, the more likely the event.