Math Problem Statement

In the United States, voters who are neither Democrat nor Republican are called Independent. It is believed that 11% of voters are Independent. A survey asked 33 people to identify themselves as Democrat, Republican, or Independent.

A. What is the probability that none of the people are Independent?

Probability = 

B. What is the probability that fewer than 7 are Independent?

Probability = 

C. What is the probability that more than 2 people are Independent?

Probability =

Solution

To solve this problem, we'll use the binomial distribution where:

  • The number of trials n=33n = 33 (the number of surveyed voters).
  • The probability of a voter being Independent p=0.11p = 0.11.
  • The probability of not being Independent q=1p=0.89q = 1 - p = 0.89.

A. Probability that none of the people are Independent

This is P(X=0)P(X = 0):

P(X=0)=(330)(0.11)0(0.89)33=11(0.89)33P(X = 0) = \binom{33}{0} (0.11)^0 (0.89)^{33} = 1 \cdot 1 \cdot (0.89)^{33}

Calculating (0.89)33(0.89)^{33}:

(0.89)330.0312(0.89)^{33} \approx 0.0312

So,

P(X=0)0.0312P(X = 0) \approx 0.0312

B. Probability that fewer than 7 are Independent

This is P(X<7)P(X < 7), which can be calculated as:

P(X<7)=P(X=0)+P(X=1)+P(X=2)+P(X=3)+P(X=4)+P(X=5)+P(X=6)P(X < 7) = P(X = 0) + P(X = 1) + P(X = 2) + P(X = 3) + P(X = 4) + P(X = 5) + P(X = 6)

Calculating each term:

  1. For k=1k = 1: P(X=1)=(331)(0.11)1(0.89)32=330.11(0.89)32330.110.03510.1277P(X = 1) = \binom{33}{1} (0.11)^1 (0.89)^{32} = 33 \cdot 0.11 \cdot (0.89)^{32} \approx 33 \cdot 0.11 \cdot 0.0351 \approx 0.1277

  2. For k=2k = 2: P(X=2)=(332)(0.11)2(0.89)31=333220.0121(0.89)315280.01210.03940.2472P(X = 2) = \binom{33}{2} (0.11)^2 (0.89)^{31} = \frac{33 \cdot 32}{2} \cdot 0.0121 \cdot (0.89)^{31} \approx 528 \cdot 0.0121 \cdot 0.0394 \approx 0.2472

  3. For k=3k = 3: P(X=3)=(333)(0.11)3(0.89)30=33323160.001331(0.89)3054560.0013310.04420.2422P(X = 3) = \binom{33}{3} (0.11)^3 (0.89)^{30} = \frac{33 \cdot 32 \cdot 31}{6} \cdot 0.001331 \cdot (0.89)^{30} \approx 5456 \cdot 0.001331 \cdot 0.0442 \approx 0.2422

  4. For k=4k = 4: P(X=4)=(334)(0.11)4(0.89)29359600.000146(0.89)29359600.0001460.04950.0326P(X = 4) = \binom{33}{4} (0.11)^4 (0.89)^{29} \approx 35960 \cdot 0.000146 \cdot (0.89)^{29} \approx 35960 \cdot 0.000146 \cdot 0.0495 \approx 0.0326

  5. For k=5k = 5: P(X=5)=(335)(0.11)5(0.89)282373360.0000161(0.89)280.0022P(X = 5) = \binom{33}{5} (0.11)^5 (0.89)^{28} \approx 237336 \cdot 0.0000161 \cdot (0.89)^{28} \approx 0.0022

  6. For k=6k = 6: P(X=6)=(336)(0.11)6(0.89)2713459680.000001771(0.89)270.0003P(X = 6) = \binom{33}{6} (0.11)^6 (0.89)^{27} \approx 1345968 \cdot 0.000001771 \cdot (0.89)^{27} \approx 0.0003

Now summing these probabilities:

P(X<7)0.0312+0.1277+0.2472+0.2422+0.0326+0.0022+0.00030.6832P(X < 7) \approx 0.0312 + 0.1277 + 0.2472 + 0.2422 + 0.0326 + 0.0022 + 0.0003 \approx 0.6832

C. Probability that more than 2 people are Independent

This is P(X>2)=1P(X2)P(X > 2) = 1 - P(X \leq 2).

Calculating P(X2)P(X \leq 2):

P(X2)=P(X=0)+P(X=1)+P(X=2)0.0312+0.1277+0.24720.4061P(X \leq 2) = P(X = 0) + P(X = 1) + P(X = 2) \approx 0.0312 + 0.1277 + 0.2472 \approx 0.4061

Thus,

P(X>2)=10.40610.5939P(X > 2) = 1 - 0.4061 \approx 0.5939

Summary of Results

A. Probability that none are Independent: P(X=0)0.031P(X = 0) \approx 0.031

B. Probability that fewer than 7 are Independent: P(X<7)0.683P(X < 7) \approx 0.683

C. Probability that more than 2 are Independent: P(X>2)0.594P(X > 2) \approx 0.594

Would you like more details or have any questions? Here are some related questions you might consider:

  1. How would the probabilities change if the percentage of Independents were to rise to 20%?
  2. What assumptions are made when using the binomial distribution in this context?
  3. How does increasing the sample size affect the probabilities in a binomial distribution?
  4. Can you explain the significance of the complement rule in probability?
  5. What are some practical implications of these probability findings for political campaigns?

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Math Problem Analysis

Mathematical Concepts

Probability
Binomial Distribution

Formulas

P(X = k) = (n choose k) * p^k * (1 - p)^(n - k)
P(X < k) = sum of probabilities from P(X = 0) to P(X = k - 1)
Complement Rule: P(X > k) = 1 - P(X ≤ k)

Theorems

Binomial Theorem
Complement Rule in Probability

Suitable Grade Level

Grades 11-12