The document is calculating the probability that 3 people selected at random from a class of 16 people will not have birthdays in October. There are 4 people in the class with October birthdays, and the number of ways to select 3 people not having October birthdays is 220. The probability is calculated as 220 over the total number of ways to select 3 people from the class, which is 11/28.

1. a class of 16 people contains, 4 people whose birthday is in October, if 3 people from the class
are selected at random, find the probability that none of those selected has an October birthday
p(3 none October birthday)
Solution
Number of ways 3 people from the class are selected at random that none of those selected has
an October birthday = 12C3 =220
Probability = 220/16C3
=11/28