The number of ways so that the birthdays of 6 people falls in exactly ...
No. of ways to choose the 3 months = 12C3 = 220
Now, we have 6 people and 3 months for the birthdays to fall in.
Since none of these three months can have no birthday in it, the 3 possible ways of distributing the 6 people in 3 months would be (4, 1, 1), (3, 2, 1), (2, 2, 2)
(4, 1, 1)
No. of ways of rotating the distribution of the number of birthdays in these three months = 3!/2!
No. of ways of rotating the distribution of the people = 6C4 . 2C1 . 1C1
Therefore, the total number of ways in (4, 1, 1) case
= (3!/21).6C4 . 2C1 . 1C1 = 90 ways
(3, 2, 1)
No. of ways of rotating the distribution of the number of birthdays in these three months = 3!
No. of ways of rotating the distribution of the people = 6C4 . 2C1 . 1C1
Therefore, the total number of ways in (3, 2, 1) case
3!.6C4 . 2C1 . 1C1 = 360 ways
(2, 2, 2)
No. of ways of rotating the distribution of the number of birthdays in these three months
= 3!/3!
No. of ways of rotating the distribution of the people = 6C4 . 2C1 . 1C1
Therefore, total number of ways in (2, 2, 2) case =(3!/3!).6C4 . 2C1 . 1C1 = 90
The number of ways so that the birthdays of 6 people falls in exactly 3 calendar months = (90 + 360 + 90) • 220 = 118800