7.1 Probability of an Event


7.1 Probability of an Event

The probability of an event A, P(A) is given by

  P(A)= Number of times event A occurs Number of trials      P(A)= n(A) n(S)   where 0P(A)1  

If P(A) = 0, then the event A will certainly not occur.
If P(A) = 1, then the event A is sure to occur.


Example 1:
A box contains 9 red pens and 13 blue pens. Tom puts another 4 red pens and 2 blue pens into the box. A pen is picked at random from the box. What is the probability that a red pen is picked?

Solution:
n(S) = 9 + 13 + 4 + 2 = 28
Let A = Event that a red pen is picked
n(A) = 9 + 4 = 13
P( A )= n( A ) n( S )           = 13 28


Example 2:
A bag contains 45 green cards and yellow cards. A card is picked at random from the bag. The probability that a green card is picked is 1 5  .  How many green cards must be added to the bag so that the probability of picking a green card becomes ½?

Solution:
n(S) = 45
Let
x = number of green cards in the bag.
A = Event of randomly picking a green card.
n(A) = x
P( A )= n( A ) n( S )       1 5  = x 45       x= 45 5       x=9

Let y is the number of green cards added to the bag.
9+y 45+y = 1 2
2 (9 + y) = 45 + y
 18 + 2y = 45 + y
    2yy = 45 – 18
            y = 27