DEFINITION If there are n elementary events associated with a random experiment and m of them are favourable to an event A, then the probability of happening or occurrence of event
A is denoted by P (A) and is defined as the ratio m/n
Thus , P (A) = m/n = Favourable number of elementary events/ Total number of elementary events
(v) If any of the elementary events HHH, HHT, HTH, and THH is an outcome, then we say that the event “Getting at least two heads” occurs. Favourable number of elementary events = 4
Hence, required probability = 4/8 = ½
Example : A bag contains 3 red and 2 blue marbles. A marble is drawn at random. What is the probability of drawing a blue marble?
Solution
There are 5 marbles in the bag. Out of these 5 marbles one can be chosen in 5 ways.
Total number of elementary events = 5
Since the bag contains 2 blue marbles. Therefore, one blue marble can be drawn in 2 ways.
Favourable number of elementary events = 2
Hence, P (Getting a blue marble) = 2/5