Skoool Kerala

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 of an event A happening is given by this formula: 

Equally Likely Outcomes 

When a dice is thrown and the number shown is recorded, there are six possible results: 1, 2, 3, 4, 5, 6. 
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 

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