An unbiased dice, with faces numbered 1,2,3,4,5,6, is thrown `n` times and the list of `n` numbers shown up is noted. Then find the probability that among the numbers 1,2,3,4,5,6 only three numbers appear in this list.

Explanation

When a dice is rolled n times, total number of cases is `6^(n)` Let us first select three numbers which appear.
Three numbers can be selected from six `.^(6)C_(3)` ways. Now each of these three numbers appear at least once. This is same as filling three different boxes with n different objects such that no box remains empty.
Number of ways of such distribution using the principle of inclusion-exclusion = `3^(n) - .^(3)C_(1)(3 - 1)^(n ) + .^(3)C_(2) (3 - 2)^(n)`
`= 3^(n) - 3 xx 2^(n) + 3`
`therefore` Required probability = `((3^(n) - 3 xx 2^(n) + 3) xx .^(6)C_(3))/(6^(n))`