Question #330760

Renzo tossed a pair of dice five times. What is the probability that he will get at least three "9"

1
Expert's answer
2022-04-20T02:47:59-0400

The total number of possible outcomes for a single toss of a pair of dice N=66=36.N=6\cdot6=36.

The favorable outcomes (Renzo gets 9) are (3, 6), (4, 5), (5, 4), (6, 3); n=4.n=4.

The probability of getting a 9 for each toss is p=436=19.p=\cfrac{4}{36}=\cfrac{1}{9}.

The probability of getting another sum is q=119=89.q=1-\cfrac{1}{9}=\cfrac{8}{9}.

We have a Bernoulli trial - exactly two possible outcomes, "success" (Renzo gets a 9) and "failure" (Renzo gets another sum) and the probability of success is the same every time the experiment is conducted (Renzo tosses a pair of dice).

The probability of each result

P(X=k)=(nk)pkqnk==(5k)(19)k(89)5k==5!k!(5k)!(19)k(89)5k.P(X=k)=\begin{pmatrix}n\\k\end{pmatrix}\cdot p^k\cdot q^{n-k}=\\ =\begin{pmatrix}5\\k\end{pmatrix}\cdot \begin{pmatrix}\cfrac{1}{9}\end{pmatrix}^{k}\cdot \begin{pmatrix}\cfrac{8}{9}\end{pmatrix}^{5-k}=\\ =\cfrac{5!}{k!\cdot(5-k)!}\cdot \begin{pmatrix}\cfrac{1}{9}\end{pmatrix}^{k}\cdot \begin{pmatrix}\cfrac{8}{9}\end{pmatrix}^{5-k}\\.

The sought probability that Renzo gets at least three 9:

P(X3)==P(X=3)+P(X=4)+P(X=5)==5!3!2!(19)3(89)2+5!4!1!(19)4(89)1++5!5!0!(19)5(89)0==0,011533.P(X\geq 3)=\\ =P(X=3)+P(X=4)+P(X=5)=\\ =\cfrac{5!}{3!\cdot2!}\cdot \begin{pmatrix}\cfrac{1}{9}\end{pmatrix}^{3}\cdot \begin{pmatrix}\cfrac{8}{9}\end{pmatrix}^2+\cfrac{5!}{4!\cdot1!}\cdot \begin{pmatrix}\cfrac{1}{9}\end{pmatrix}^{4}\cdot \begin{pmatrix}\cfrac{8}{9}\end{pmatrix}^1+\\ +\cfrac{5!}{5!\cdot0!}\cdot \begin{pmatrix}\cfrac{1}{9}\end{pmatrix}^{5}\cdot \begin{pmatrix}\cfrac{8}{9}\end{pmatrix}^0=\\ =0,011533.

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