Independent Events and Unions of Events
Help Questions
AP Statistics › Independent Events and Unions of Events
In a survey, 48% of respondents prefer Brand X, 42% prefer Brand Y, and 10% prefer both (they said either would be fine). Let Event A be “a randomly selected respondent prefers Brand X” and Event B be “the respondent prefers Brand Y.” Which statement about Events A and B is correct?
Events A and B are mutually exclusive, so $P(A\cup B)=P(A)+P(B)$.
Events A and B are independent because $P(A\cup B)=P(A)+P(B)+P(A\cap B)$.
Events A and B are independent because “prefers X” and “prefers Y” are different options.
Events A and B are not independent because $P(A\cap B)$ is not equal to $P(A)P(B)$.
Events A and B are independent because $P(A\cap B)$ is 10%.
Explanation
This question tests independence with overlapping preferences. Given P(A) = 0.48, P(B) = 0.42, and P(A∩B) = 0.10, we check if P(A∩B) = P(A)·P(B). We calculate P(A)·P(B) = 0.48 × 0.42 = 0.2016. Since 0.10 ≠ 0.2016, events A and B are not independent. The lower intersection probability (0.10 < 0.2016) indicates negative association - people tend to prefer one brand or the other, not both. This shows that events representing choices or preferences often exhibit dependence, as people's preferences tend to be exclusive rather than independent.
A bag contains 5 white and 5 black chips. Two chips are drawn with replacement. Let Event A be “the first chip is white” and Event B be “the second chip is white.” Which statement about Events A and B is correct?
Events A and B are not independent because the same bag is used twice.
Events A and B are independent only if the first chip is black.
Events A and B are not independent because A and B can both occur.
Events A and B are mutually exclusive because only one chip can be white.
Events A and B are independent because replacing the first chip keeps the second-draw probabilities the same.
Explanation
This question contrasts sampling with and without replacement. When drawing with replacement, each draw is from the same distribution: P(white) = 5/10 = 1/2 for both draws. Events A and B are independent because P(B|A) = P(B|A') = P(B) = 1/2 - the first draw's outcome doesn't affect the second draw's probabilities since the chip is replaced. We can verify: P(A∩B) = P(A)·P(B) = (1/2)·(1/2) = 1/4. The replacement ensures that each draw is independent, maintaining the same probability distribution throughout the experiment.
A jar contains 25 candies. Of these, 15 are chocolate (C), 10 are fruit (not chocolate), and 8 candies are individually wrapped (W). Among the 15 chocolate candies, 3 are wrapped; among the 10 fruit candies, 5 are wrapped. One candy is selected at random. Let Event A be “the candy is chocolate” and Event B be “the candy is wrapped.” Which statement about Events A and B is correct?
Events A and B are independent because $P(A\cap B)=P(A)+P(B)$.
Events A and B are not independent because chocolate candies and wrapped candies cannot overlap.
Events A and B are not independent because $P(A\cap B)\ne P(A)P(B)$.
Events A and B are independent because $P(A\cup B)=P(A)+P(B)$.
Events A and B are independent because there are wrapped candies in both categories.
Explanation
This problem involves calculating independence with overlapping categories. From the given information: P(A) = 15/25, P(B) = 8/25, and P(A∩B) = 3/25 (wrapped chocolate candies). To check independence: P(A)·P(B) = (15/25)·(8/25) = 120/625 = 24/125, while P(A∩B) = 3/25 = 15/125. Since 24/125 ≠ 15/125, events A and B are not independent. Choice E incorrectly claims chocolate and wrapped candies cannot overlap, but the problem explicitly states 3 chocolate candies are wrapped. The calculation shows these events are dependent—knowing a candy is chocolate changes the probability it's wrapped.
At a school carnival, a student randomly selects one ticket from a box containing 40 tickets: 18 are blue, 12 are red, and 10 are green. Some tickets are marked as “prize” tickets: 14 of the 40 tickets are prize tickets, including 6 blue prize tickets, 5 red prize tickets, and 3 green prize tickets. Let Event A be “the ticket is blue” and Event B be “the ticket is a prize ticket.” Which statement about Events A and B is correct?
Events A and B are independent because $P(A\cup B)=P(A)+P(B)$.
Events A and B are independent because a ticket cannot be both blue and a prize ticket.
Events A and B are not independent because $P(A\cap B)\ne P(A)P(B)$.
Events A and B are not independent because Events A and B overlap.
Events A and B are independent because $P(A\cap B)=P(A)P(B)$.
Explanation
This question tests understanding of independent events versus mutually exclusive events. To check if events A and B are independent, we need to verify if P(A∩B) = P(A)·P(B). From the given information: P(A) = 18/40, P(B) = 14/40, and P(A∩B) = 6/40 (blue prize tickets). Calculating: P(A)·P(B) = (18/40)·(14/40) = 252/1600 = 63/400, while P(A∩B) = 6/40 = 60/400. Since 63/400 ≠ 60/400, the events are not independent. Choice A incorrectly confuses independence with mutual exclusivity—the fact that some tickets are both blue AND prize tickets shows they're not mutually exclusive.
In a survey of 200 students, 90 reported that they play a sport (S), 120 reported that they have a part-time job (J), and 50 reported both playing a sport and having a part-time job. One student is selected at random. Let Event A be “the student plays a sport” and Event B be “the student has a part-time job.” Which statement about Events A and B is correct?
Events A and B are independent because $P(A\cup B)=P(A)+P(B)$.
Events A and B are independent because playing a sport implies having a job.
Events A and B are not independent because $P(A\cap B)\ne P(A)P(B)$.
Events A and B are independent because $P(A\cap B)=P(A)P(B)$.
Events A and B are not independent because the events overlap.
Explanation
This problem tests independence with survey data. Given: P(A) = 90/200, P(B) = 120/200, and P(A∩B) = 50/200. For independence, we need P(A∩B) = P(A)·P(B). Calculating: P(A)·P(B) = (90/200)·(120/200) = 10800/40000 = 54/200. However, P(A∩B) = 50/200. Since 54/200 ≠ 50/200, events A and B are not independent. Choice C incorrectly states that overlapping events cannot be independent—events can overlap and still be independent if they satisfy the multiplication rule. The key is checking the probability relationship, not just whether events can occur together.
A school reports that 40% of students play a sport, 30% are in band, and 15% do both. Let Event A be “a randomly selected student plays a sport” and Event B be “the student is in band.” Which statement about Events A and B is correct?
Events A and B are mutually exclusive because some students do both.
Events A and B are independent because $P(A\cap B)$ is less than both $P(A)$ and $P(B)$.
Events A and B are independent because $P(A\cup B)=P(A)+P(B)$.
Events A and B are not independent because $P(A\cap B)$ is not equal to $P(A)P(B)$.
Events A and B are independent because band and sports are different activities.
Explanation
This question tests the independence formula P(A∩B) = P(A)·P(B) with given probabilities. We have P(A) = 0.40, P(B) = 0.30, and P(A∩B) = 0.15. For independence, we need P(A∩B) = P(A)·P(B) = 0.40 × 0.30 = 0.12. Since 0.15 ≠ 0.12, events A and B are not independent. The fact that P(A∩B) > P(A)·P(B) indicates positive association - students who play sports are more likely than average to be in band. This demonstrates that independence requires the exact equality P(A∩B) = P(A)·P(B), not just that events can occur together.
A company has 80 employees. Of these, 30 work remotely (R), 28 are in the sales department (S), and 12 are both remote and in sales. One employee is selected at random. Let Event A be “the employee works remotely” and Event B be “the employee is in sales.” Which statement about Events A and B is correct?
Events A and B are independent because remote work and sales are unrelated job features.
Events A and B are independent because 12 employees are in both categories.
Events A and B are not independent because $P(A\cap B)\ne P(A)P(B)$.
Events A and B are not independent because Events A and B are mutually exclusive.
Events A and B are independent because $P(A\cup B)=P(A)+P(B)$.
Explanation
This question involves checking independence using given frequencies. From the data: P(A) = 30/80, P(B) = 28/80, and P(A∩B) = 12/80. To verify independence, we check if P(A∩B) = P(A)·P(B). Calculating: P(A)·P(B) = (30/80)·(28/80) = 840/6400 = 21/160, while P(A∩B) = 12/80 = 24/160. Since 21/160 ≠ 24/160, the events are not independent. The fact that 12 employees are in both categories (choice A) doesn't determine independence—we must check if the probability relationship holds. Events that overlap can still be independent if they satisfy the multiplication rule.
A jar contains 25 candies. Of these, 15 are chocolate (C), 10 are fruit (not chocolate), and 8 candies are individually wrapped (W). Among the 15 chocolate candies, 3 are wrapped; among the 10 fruit candies, 5 are wrapped. One candy is selected at random. Let Event A be “the candy is chocolate” and Event B be “the candy is wrapped.” Which statement about Events A and B is correct?
Events A and B are not independent because chocolate candies and wrapped candies cannot overlap.
Events A and B are independent because $P(A\cap B)=P(A)+P(B)$.
Events A and B are independent because $P(A\cup B)=P(A)+P(B)$.
Events A and B are independent because there are wrapped candies in both categories.
Events A and B are not independent because $P(A\cap B)\ne P(A)P(B)$.
Explanation
This problem involves calculating independence with overlapping categories. From the given information: P(A) = 15/25, P(B) = 8/25, and P(A∩B) = 3/25 (wrapped chocolate candies). To check independence: P(A)·P(B) = (15/25)·(8/25) = 120/625 = 24/125, while P(A∩B) = 3/25 = 15/125. Since 24/125 ≠ 15/125, events A and B are not independent. Choice E incorrectly claims chocolate and wrapped candies cannot overlap, but the problem explicitly states 3 chocolate candies are wrapped. The calculation shows these events are dependent—knowing a candy is chocolate changes the probability it's wrapped.
A card is drawn at random from a standard 52-card deck. Let Event A be “the card is a heart” and Event B be “the card is a face card (J, Q, or K).” Which statement about Events A and B is correct?
Events A and B are not independent because $P(A\cup B)=P(A)+P(B)+P(A\cap B)$.
Events A and B are mutually exclusive because a heart cannot be a face card.
Events A and B are independent because suit and rank are unrelated categories.
Events A and B are not independent because $P(A\cap B)=P(A)+P(B)$.
Events A and B are independent because $P(A\cap B)=P(A)P(B)$.
Explanation
This question tests identifying independent events in a card draw by verifying P(A ∩ B) = P(A)P(B). Event A (heart) has P(A) = 13/52 = 0.25, Event B (face card) has P(B) = 12/52 ≈ 0.231, and P(A ∩ B) = 3/52 ≈ 0.0577, equaling 0.25 * 0.231 ≈ 0.0577, confirming independence. The relationship reflects that suit and rank are unrelated in a standard deck. Distractor choice B confuses independence with exclusivity, as hearts can be face cards. Choice D uses the union formula incorrectly as a test for independence. Mini-lesson: Independence is confirmed when the intersection probability is the product of individuals, unlike mutual exclusivity where the intersection is zero; these are distinct, as independent events can overlap, while exclusive ones cannot.
A fair six-sided die is rolled once. Let Event A be “the result is even” and Event B be “the result is greater than 4.” Which statement about Events A and B is correct?
Events A and B are independent because $P(A\cup B)=P(A)+P(B)$.
Events A and B are not independent because $P(A\cap B)=0$.
Events A and B are independent because “even” and “greater than 4” describe different properties.
Events A and B are not independent because they overlap.
Events A and B are independent because $P(A\cap B)=P(A)P(B)$.
Explanation
This problem tests calculating independence for events with a fair die. Event A (even) includes {2, 4, 6}, so P(A) = 3/6 = 1/2. Event B (greater than 4) includes {5, 6}, so P(B) = 2/6 = 1/3. The intersection A∩B = {6}, so P(A∩B) = 1/6. Checking independence: P(A)·P(B) = (1/2)·(1/3) = 1/6 = P(A∩B). Since the equation holds, events A and B are independent. Choice C incorrectly applies the addition rule for mutually exclusive events, but these events overlap at outcome 6. Independence means knowing one event occurred doesn't change the probability of the other, which is true here.