Intro to Probability


What is Probability?

Probability is simply how likely something is to happen.

The probability of event “A” happening is:

The easiest way to understand probability is with some examples.

Probability Examples

Example 1: You flip a coin once. What is the probability that it lands on heads?

The total number of outcomes is two – heads or tails.

The total number of ways it can land on heads is one. 

So the probability that it lands on heads is:


Example 2: You roll a die once. What is the probability of rolling a three?

The total number of outcomes is six – you could roll a 1, 2, 3, 4, 5, or 6.

The total number of ways you could roll a three is simply one. 

So the probability of rolling a three is:


Example 3: You roll a die once. What is the probability of rolling a three or a five?

The total number of outcomes is still six – you could roll a 1, 2, 3, 4, 5, or 6.

The total number of ways you could roll a three or a five is two – you could roll a three or a five.

So the probability of rolling a three or a five is:


Example 4: You roll a die once. What is the probability of rolling an odd number?

The total number of outcomes is still six – you could roll a 1, 2, 3, 4, 5, or 6.

The total number of ways you could roll an odd number is three – you could roll a 1, 3, or 5.

So the probability of rolling an odd number is:


Summary

The probability that event “A” happens is written as P(A).

The probability of an event can only be between 0 and 1 and it can be written as a fraction or a percentage.

The higher the probability, the more likely an event is to occur. The lower the probability, the less likely an event is to occur.

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