Finding the sum of probabilities

Note: the sum of probabilities should equal to 1.

The probability of tossing a head of a coin is \frac { 1 }{ 2 } and the probability of tossing a tail of a coin is \frac { 1 }{ 2 }\frac { 1 }{ 2 } +\frac { 1 }{ 2 } =1.

If you go to school, the probability of you being early is \frac { 3 }{ 8 } what is the probability that you will be late?

1-\frac { 3 }{ 8 } =\frac { 5 }{ 8 }

To work out the probability of an outcome:


For example

To work out the probability of rolling an even number on an ordinary dice, find out how many ways you can do that. There are 3 different even numbers out of a total of 6 numbers, so the probability is \frac { 3 }{ 6 } which is \frac { 1 }{ 2 }.

Events that can’t occur at the same time are called ‘mutually exclusive’. For example, you can’t win, lose and draw in a game of netball, so these events are mutually exclusive.

Relative frequency is the number of times an event occurs, divided by the total number of trials in an experiment.

This provides an estimation when the theoretical probability cannot be used.