When we talk about probability, we are talking about how likely something is to happen, measured on a scale from 0 to 1.

A probability of 0 means that it is impossible that something will happen. So, you could say that the probability that you will grow to 100m tall is 0.

And if something has a probability of 1 then it will certainly happen. So the probability that it will rain somewhere in the world this year is 1.

Most events have a probability between 0 and 1, such as 1/2 or 0.2. We can express these probabilities either as a fraction or a decimal (we don’t often express them as percentages in mathematics classes, but it is also ok to do that).

Experimental Probability

We can calculate the probability of certain things happening by doing experiments lots of times. For instance, if I toss a coin 1,000,000 times and find that 500,000 times it lands on heads, I can make a reasonable estimate that the probability of it landing on heads is 1/2. (What would you think if it landed on heads 250,000 times? What about if it landed on heads 499,999 times?

We can use a formula to express this, as follows:

Example

Exercise

Now let’s try questions 5 to 8 of exercise 18B on page 288:

The answers are below: