Question

Find the variance of the random variable in Exercise 51 of Chapter Four.

51. Four letters are drawn, one at a time, from WALLA WALLA. What is the expected number of different letters to be drawn?

Answer 1

Solution:-

Actually WALLA WALLA includes 10 letters furthermore, 3 letters of same kind .

Here four letters are drawn among 2 W, 4 A, 4 L (since it has taken from  WALLA WALLA )

Expected no.of various letters that is letter once chosen is no more chose.

What's more, henceforth for 4 places four letters would be chose = n(n-1)(n-2)(n-3)

The expected number of different letters to be drawn = 10*9*8*7

The expected number of different letters to be drawn = 5040