A fair die is rolled eight times. Find the probability of each event.
Recall that the binomial probability is represented by the equation
$c(n,r) (p)^r (q)^{n-r}$
a.) A six occurs four times.
The probability that a dies shows a six or the probability of success $p$ is $\displaystyle \frac{1}{6}$. On the other hand, the probability that a die do not show a $6$ or the probability of failure is $\displaystyle q=1-p = \frac{5}{6}$. Thus, the probability in this case is
$\displaystyle = C(8,4) \left( \frac{1}{6} \right)^4 \left( \frac{5}{6} \right)^{8-4}$
$= 0.026$
b.) An even number occurs two or more times.
The probability that a die shows an even number or the probability of success $p$ is $\displaystyle \frac{3}{6} = \frac{1}{2}$. On the other hand, the probability that a die do not show an even number is $\displaystyle q=1-p = \frac{1}{2}$. To solve this in a faster way, we can apply the compliment to the probability that even number occurs once or never. In this case, we have
$
\begin{equation}
\begin{aligned}
=& 1 - \left[ C(8,11) \left( \frac{1}{2} \right)^1 \left( \frac{1}{2} \right)^{8-1} + C(8,0) \left( \frac{1}{2} \right)^0 \left( \frac{1}{2} \right)^{8-0} \right]
\\
\\
=& 1 - [0.3721 + 0.1667]
\\
\\
=& 0.4612
\end{aligned}
\end{equation}
$
Sunday, September 11, 2016
College Algebra, Chapter 10, Review Exercises, Section Review Exercises, Problem 40
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