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: