[Math] A fair die is rolled five times. Find the probability that one shows twice, three shows twice, and six shows once.

probabilityprobability theoryrandom variables

I am working on the following problem:

A fair die is rolled five times. Find the probability that one shows twice, three shows twice, and six shows once.

The answer in the solution is given : $$\left(\frac{5!}{1! 2! 2!}\right) \left(\frac{1}{6}\right)^6$$

But I think it should be $(1/6)^5$.

What it the correct answer?

(Ref: Probability, Random variables and Stochastic processes – Papulis(Ch-4/Exercise-31)

Best Answer

Since the die is fair, there are $6$ equally likely outcomes for each of the five rolls, so there are $6^5$ possible outcomes for the five rolls.

We have a sequence of five outcomes. For the favorable cases, choose two of the five positions in the sequence for the ones and two of the remaining three positions in the sequence for the threes. The final open position in the sequence must be filled with a six. Therefore, there are $$\binom{5}{2}\binom{3}{2} = \frac{5!}{3!2!} \cdot \frac{3!}{2!1!} = \frac{5!}{2!2!1!}$$ favorable sequences.

Hence, the probability that two ones, two threes, and a six are obtained when a fair die is rolled five times is $$\frac{\dbinom{5}{2}\dbinom{3}{2}}{6^5} = \left(\frac{5!}{2!2!1!}\right)\left(\frac{1}{6^5}\right)$$