Probability 1

probability Probability of an event

Probability The PROBABILITY of an event is the ratio of the number of times an event occurs to the number of trials. P (A) = n (A) n (S)

Example 1 There

are 45 students in form 4 ceria, of which 25 are girls. A student is selected at random from the class. Find the probability that the selected student is a girl.

Solution: The total number of students = 45 The number of girls = 25 Let A be the event of selecting a girl P(A) =

number of girls in form 4 ceria total number of students in form 4 ceria = 25 45 = 5 9

Example 2

Solution ;  The

number of blue marbles = 180 –

100 = 80 • P ( selecting a blue marble ) =

80 100 = 4 9

   

Conclusion… Probability of event A, P(A)= num.of times event A occur number of trials If A will not occur, then P(A) = 0 If A occurs in every trial, then P(A) = 1

In

general P(A) = n(A)  n(S) n(A) = P(A) x n(S)