Math Problem Statement

In a survey of U.S. adults with a sample size of

20052005​,

322322

said Franklin Roosevelt was the best president since World War II.

TwoTwo

U.S. adults are selected at random from this sample without replacement. Complete parts​ (a) through​ (d).

Question content area bottom

Part 1

​(a) Find the probability that both adults say Franklin Roosevelt was the best president since World War II.

The probability that both adults say Franklin Roosevelt was the best president since World War II is

0.0260.026.

​(Round to three decimal places as​ needed.)

Part 2

​(b) Find the probability that neither adult says Franklin Roosevelt was the best president since World War II.

The probability that neither adult says Franklin Roosevelt was the best president since World War II is

0.70.7.

​(Round to three decimal places as​ needed.)

Solution

Given:

  • Total sample size = 20052005
  • Number of people who said Franklin Roosevelt was the best president = 322322
  • Two adults are selected without replacement.

Part (a) Find the probability that both adults say Franklin Roosevelt was the best president since World War II:

  1. The probability of the first adult saying Franklin Roosevelt was the best president is:

    P(first adult says Roosevelt)=3222005P(\text{first adult says Roosevelt}) = \frac{322}{2005}

  2. After the first adult is selected, there are now 3221=321322 - 1 = 321 Roosevelt supporters left and a total of 20051=20042005 - 1 = 2004 adults left in the sample. The probability that the second adult also says Roosevelt is:

    P(second adult says Roosevelt)=3212004P(\text{second adult says Roosevelt}) = \frac{321}{2004}

  3. Therefore, the probability that both adults say Franklin Roosevelt was the best president is:

    P(both say Roosevelt)=3222005×3212004P(\text{both say Roosevelt}) = \frac{322}{2005} \times \frac{321}{2004}

    P(both say Roosevelt)0.026P(\text{both say Roosevelt}) \approx 0.026

Thus, the probability is approximately 0.026.


Part (b) Find the probability that neither adult says Franklin Roosevelt was the best president since World War II:

  1. The probability of the first adult not saying Franklin Roosevelt was the best president is:

    P(first adult not Roosevelt)=20053222005=16832005P(\text{first adult not Roosevelt}) = \frac{2005 - 322}{2005} = \frac{1683}{2005}

  2. After selecting the first adult, there are 20051=20042005 - 1 = 2004 people left, and 16831=16821683 - 1 = 1682 adults left who do not support Roosevelt. The probability that the second adult also does not say Roosevelt is:

    P(second adult not Roosevelt)=16822004P(\text{second adult not Roosevelt}) = \frac{1682}{2004}

  3. Therefore, the probability that neither adult says Franklin Roosevelt was the best president is:

    P(neither says Roosevelt)=16832005×16822004P(\text{neither says Roosevelt}) = \frac{1683}{2005} \times \frac{1682}{2004}

    P(neither says Roosevelt)0.7P(\text{neither says Roosevelt}) \approx 0.7

Thus, the probability is approximately 0.7.

Would you like further clarification or details on any of these steps?


Here are 5 related questions to expand on this topic:

  1. What is the probability that exactly one of the two selected adults says Franklin Roosevelt was the best president?
  2. How does the probability change if the sample is much smaller, say 500 adults?
  3. How would the probability change if the selection was with replacement?
  4. What is the expected number of Roosevelt supporters in a randomly selected group of 10 people?
  5. Can we use a binomial distribution approximation for this problem? Why or why not?

Tip: When selecting items without replacement, the probabilities change after each selection. This is different from cases with replacement, where probabilities remain constant.

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Math Problem Analysis

Mathematical Concepts

Probability
Combinatorics

Formulas

P(first adult says Roosevelt) = 322 / 2005
P(second adult says Roosevelt) = 321 / 2004
P(both say Roosevelt) = (322 / 2005) * (321 / 2004)
P(first adult not Roosevelt) = 1683 / 2005
P(second adult not Roosevelt) = 1682 / 2004
P(neither says Roosevelt) = (1683 / 2005) * (1682 / 2004)

Theorems

Multiplication Rule of Probability

Suitable Grade Level

Grades 10-12