Probability basics

Last updated: 18 Jul 2007

Probability is a measure of how likely a particular outcome to an event is to happen.

It ranges from 0 to 1. A probability of 0 means that the outcome cannot happen. A probability of 1 means that the outcome will definitely happen. And in between 0 and 1 means that the outcome may happen.

Example with a coin

When a coin is tossed the outcome (or event) can be heads or tails. What is the probability it is tails?

Since each outcome, heads or tails, is equally likely we can say that the probability of each is 0.5.

`P(text{coin toss is tails}) = 1/2`

Basic rule of probability

More generally we can say that where there are n equally likely outcomes then the probability of each of these possibilities will be `1/n`.

So we can say that

`P(text{outcome}) = text{number of ways it can happen} / text{total number of possible outcomes}`

This is the basis of all probability questions in the GMAT.

Example with die

What is the probability of rolling a 6 when you throw a 6 sided die.

Each number from 1 to 6 is equally likely to be thrown and only one of those outcomes is a 6 so using the general rule we can say that

`P(text{throw a six}) = 1/6`

Example with cards

If you pick a card at random from a deck of cars what is the probability that it is an ace?

There are 52 cards in a pack and of those there are 4 aces so

`P(text{picking an ace}) = 4/52 = 1/13`

Next page: 'And' probabilities