An urn contains 7 white, 5 black and 3 red balls. Two balls are drawn at random. Find the probability that 9i) both the balls are red (ii) one ball is red and the other is black (iii) one ball is white.
given: urn containing 7 white, 5 black and 3 red
formula: 
two balls are drawn at random, total possible outcomes are 15C2
therefore n(S)=18C2=105
(i) let E be the event that both balls are red
n(E)= 3C2=3
(ii) let E be the event that one is red and other is black
n(E)= 3C15C1=15
(iii) let E be the event that one ball is white
n(E)= 8C17C1=56
42