Example In a class there are 13 students. 5 of them are boys
and the rest are girls. Find the probability that two students selected
at random wil be both girls.
Solution : Two students out of 13 can be selected in  ways
and two girls out
of 8 can be selected in  ways.
Therefore, required probability = 
Example A box contains 5 white balls, 4 black balls and 3 red
balls. Three balls are drawn randomly. What is the probability that they
will be (i) white (ii) black (iii) red ?
Solution : Let W, B and R denote the events of drawing three
white, three black and
three red balls respectively.
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