A rectangular box contains balls numbered {1, 2, 3, … , 18, 19, 20}. If a ball is randomly drawn from a box, what is the probability that it is a number divisible by 4 or a number divisible by 5? Solution: 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20 No. divisible by 4 – ( 4,8,12,16,20) No.divisible by 5 –( 5,10,15,20) P ( A or B) = P( A) + P ( B ) – P ( A and B) P(no. div.by 4 or no. div. by 5 ) = 5/20 + 4/20 – 1/20 = 8/20 P( no.div.by 4 or no.div.by 5 ) =2/5