A bag contains 5 red, 6 white and 7 black balls. Two balls are drawn at random. What is the probability that both balls are red or both are black?
given: bag which contains 5 red, 6 white, 7 black balls
formula: 
two balls are drawn at random
total possible outcomes are 18C2
therefore n(S)= 18C2=153
let E be the event that both balls are either red or black
n(E)=5C2+7C2=31
probability of occurrence of the event is
31