Probability for Kids: A Guide for Beginners

Probability Guide

Probability is the likelihood that an event will occur. It's often expressed as a decimal or fraction between one and zero, where zero means there's no chance the event will occur, and one means the event will definitely occur.

Simple Events

The probability that a single event will occur is equal to one over the total number of possible events. For instance, if you're rolling a 6-sided die, the probability that you'll roll a six is 1/6, since there are six possible outcomes. The probability that you'll roll any other number on the die, like a two or a four, is also 1/6.

You can also calculate the probability that one of several events will occur. For example, you might want to calculate the probability that you'll roll either a six or a three. Since you're interested in two events out of six that could possibly occur the probability is 2/6. Likewise, the probability that you'll roll a number less than six is 5/6, since five of the numbers out of the six on the die are lower than six (one through five).

Compound Events

You will also learn to evaluate the probability of multiple events happening simultaneously or over time. For example, you might want to know the probability that it will rain on both Saturday and Sunday. To calculate this, compare the number of events you're interested in to the total number of possible outcomes, just as you did with simple events.

In this example, there are four possible outcomes: rain on Saturday but not Sunday, rain on Sunday but not Saturday, rain on both days and no rain on either day. You're interested in one of them (rain on both days), so the probability is 1/4.

Reporting Probabilities

While probabilities are often reported as fractions, they can also be expressed as ratios, decimals or percentages. For instance, we can say that there's a 1/4 (one in four) chance that it will rain on both Saturday and Sunday, but we can also express that as 1:4. Yet another way to write this probability is 0.25, since 1/4 = 1 ÷ 4 = 0.25. The decimal 0.25 is equal to 25%, so we could also say there's a 25% chance that it will rain on both days.