Probability Scale and Formula

Probability is a measure of how likely an event is to happen. 
A scale is used from zero to one, as shown below.

Examples

The probability of throwing a heads with a two-headed coin is 1 (certain).
The probability of throwing a tails is 0 (impossible).
The probability of throwing a heads with a fair coin is 0,5 (evens).

Calculating probability

If all the outcomes are equally likely, then the probability an event A happening is given by this formula: 

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Equally Likely Outcomes 

When a dice is thrown and the number shown is recorded, there are six possible results:

If the dice is a perfect cube, each of these numbers is equally likely to come up. We say that there are six equally likely outcomes.

Example 1: The probability of getting an even number with a normal dice is as follows:

Example 2: The probability of getting a number greater than 4. 

Example 3: The probability of an event not happening e.g. not getting the number 6.
P (6) = 1/6 (dice) 

This is because it is a certainty that the event will or will not happen.

Example 4: If the probability of winning a tennis match is 0,3, what is the probability of losing?

P (winning at tennis) = 0,3 

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Experimental Probability 

Probability can be examined by experiment. For example, a coin is tossed 100 times and the results are recorded. 

Number of heads = 47
Number of tails = 53

The relative frequency for heads = 47/100 and for tails = 53/100. 

These are approaching the probabilities but only become accurate for a large number of trials.

Example: A drawing pin was thrown 50 times and the following results were obtained:

Landing with the sharp end down = 10 

Landing with the flat end down = 40 

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