Calculate the Probability

A bag contains 80 balls of such 20 are red, 25 are blue and 35 are white.  A ball is picked at random what is the probability that the ball picked is as:

(i)  Red ball

(ii) Black ball

(iii) Red or Blue ball.

Solution

(i) Probability of a red ball = number of red ball in the bag/total balls in the bag

= P(R)

= 20/80

= 1/4

(ii) Probability of black ball =  number of red ball in the bag/total balls in the bag

= P (B)

= 0/80

= 0 

(iii) P(R or B) = 20/80 or 25/80 = 20/80 + 25/80

=  9/16

Note: in probability or is replaced by a plus (+) sign.  Look addition rule.