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Question: A dodecahedral die has \(12\) faces that are numbered \(1\) through \(12\).

a) What is the expected value of the number that comes up when a fair dodecahedral die is rolled?

b) What is the variance of the number that comes up when a fair dodecahedral die is rolled?

Short Answer

Expert verified

Answer

a) The Expected value of the number that comes up is \(\frac{{13}}{2}\).

b) The variance of the numbers that comes up is \(\frac{{143}}{{12}}\).

Step by step solution

01

Given data

A fair dodecahedral die is rolled.

02

Concept of Probability

Probability is simply how likely something is to happen. Whenever we’re unsure about the outcome of an event, we can talk about the probabilities of certain outcomes—how likely they are. The analysis of events governed by probability is called statistics.

03

Calculation for the expected value 

a)

Let us assume \(X\) is the number that comes up when a fair dodecahedral die is rolled The variable \(X\) takes the random values \(1\;,\;2\;,\;3\;,\;4\;, \ldots \ldots \ldots \ldots \) or \(12\) with the probability \(1/12\).

Expected value is given as:

\(\begin{aligned}{}E(X) &= \frac{1}{{12}}(1 + 2 + \ldots \ldots + 12)\\E(X) &= \frac{{13}}{2}\end{aligned}\)

Therefore, the Expected value of the number that comes up is \(\frac{{13}}{2}\).

04

Calculation for the variance

b)

A fair dodecahedral die is rolled.

\(\begin{aligned}{}E\left( {{X^2}} \right) &= \frac{1}{{12}}\left( {{1^2} + {2^2} + {3^2} + {4^2} + {5^2} + \ldots \ldots .. + {{12}^2}} \right)\\E\left( {{X^2}} \right) &= \frac{{325}}{6}\end{aligned}\)

Variance is given as:

\(\begin{aligned}{}V(X) &= E\left( {{X^2}} \right) - E{()^X}\\V(X) &= \frac{{325}}{6} - {()^{\frac{{13}}{2}}}\\V(X) &= \frac{{143}}{{12}}\end{aligned}\)

Therefore, the variance of the numbers that comes up is \(\frac{{143}}{{12}}\).

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Most popular questions from this chapter

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Question: In this exercise we will use Bayes' theorem to solve the Monty Hall puzzle (Example 10 in Section 7.1). Recall that in this puzzle you are asked to select one of three doors to open. There is a large prize behind one of the three doors and the other two doors are losers. After you select a door, Monty Hall opens one of the two doors you did not select that he knows is a losing door, selecting at random if both are losing doors. Monty asks you whether you would like to switch doors. Suppose that the three doors in the puzzle are labeled 1, 2, and 3. Let W be the random variable whose value is the number of the winning door: assume that \(p\left( {W = k} \right) = \frac{1}{3}\) for k=1,2,3. Let M denote the random variable whose value is the number of the door that Monty opens. Suppose you choose door i

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e) Use Bayes' theorem to find \(p\left( {M = j\left| {M = k} \right.} \right)\) where i and j and are distinct values d) Explain why the answer to part (c) tells you whether you should change doors when Monty gives you the chance to do so,

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