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AP Statistics 4.6 Independent Events and Unions of Events- MCQs - Exam Style Questions

Question

Suppose study participants are asked two questions. Based on a large number of participants, the probability distribution for the responses to Questions 1 and 2 is given below.

  Question 1
  CorrectIncorrect
Question 2Correct0.250.20
Incorrect0.080.47

For one participant, let $A=$ Question 1 is answered correctly and $B=$ Question 2 is answered correctly. Which of the following must be true?

(A) A and B are independent because each question is correct with probability 0.50, and the probability that both questions are answered correctly is $(0.5)(0.5)=0.25$
(B) A and B are not independent because the probability in each cell is not 0.25.
(C) A and B are not independent because $(0.33)(0.45)\ne0.25$
(D) A and B are independent because one participant’s answers do not affect another participant’s answers.
(E) There is not enough information provided to determine whether A and B are independent.
▶️ Answer/Explanation
Detailed solution

Two events, A and B, are independent if the probability that they both occur is equal to the product of their individual probabilities. The rule is: $P(A \text{ and } B) = P(A) \times P(B)$.

Let’s find the probabilities from the table:
–   $P(A)$ = Probability that Question 1 is correct. We add the probabilities in the “Correct” column for Question 1: $0.25 + 0.08 = 0.33$.
–   $P(B)$ = Probability that Question 2 is correct. We add the probabilities in the “Correct” row for Question 2: $0.25 + 0.20 = 0.45$.
–   $P(A \text{ and } B)$ = Probability that both are correct. This is given directly in the table: $0.25$.

Now, let’s check if the independence rule holds:
Is $P(A) \times P(B) = P(A \text{ and } B)$?
$0.33 \times 0.45 = 0.1485$

Since $0.1485$ is not equal to $0.25$, the events are not independent. Option (C) correctly states this reasoning.
Answer: (C)

Question

Ms. Tucker travels through two intersections with traffic lights as she drives to the market. The traffic lights operate independently. The probability that both lights will be red when she reaches them is \(0.22\). The probability that the first light will be red and the second light will not be red is \(0.33\). What is the probability that the second light will be red when she reaches it?
(A) \(0.40\)
(B) \(0.45\)
(C) \(0.50\)
(D) \(0.55\)
(E) \(0.60\)
▶️ Answer/Explanation
Detailed solution

1. Define Events:
Let \(R_1\) = first light red, \(R_2\) = second light red
Given: \(P(R_1 \cap R_2) = 0.22\), \(P(R_1 \cap R_2^c) = 0.33\)

2. Find \(P(R_1)\):
\(P(R_1) = P(R_1 \cap R_2) + P(R_1 \cap R_2^c) = 0.22 + 0.33 = 0.55\)

3. Use Independence:
Since lights are independent: \(P(R_1 \cap R_2) = P(R_1) \times P(R_2)\)
\(0.22 = 0.55 \times P(R_2)\)

4. Solve for \(P(R_2)\):
\(P(R_2) = \frac{0.22}{0.55} = 0.40\)

This matches option (A).
Answer: (A)

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