Applying the Multiplication Rule for Probability

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Statistics › Applying the Multiplication Rule for Probability

Questions 1 - 10
1

A card is drawn first from a standard 52-card deck and kept out (no replacement). Event $A$ is “the first card is a heart,” and event $B$ is “the second card is a heart.” Given $P(A)=\frac{13}{52}$ and $P(B\mid A)=\frac{12}{51}$, what is the probability that both events occur, $P(A\cap B)$?

$\frac{25}{52}$

$\frac{13}{52}$

$\frac{13}{52}\cdot\frac{12}{51}=\frac{1}{17}$

$\frac{1}{4}\cdot\frac{1}{4}=\frac{1}{16}$

Explanation

This question tests the Multiplication Rule for Probability, used to find the probability of both events in a dependent sequence without replacement. It treats 'and' as sequential: first draw event A, then event B conditional on A. Here, event A is drawing a heart first from a 52-card deck, with $P(A) = \frac{13}{52}$. Given the first was a heart, the conditional probability of drawing another heart (event B) is $P(B\mid A) = \frac{12}{51}$, as one heart is removed. Multiplying yields $P(A \cap B) = \frac{13}{52} \times \frac{12}{51} = \frac{1}{17}$, the probability of drawing two hearts in sequence without replacement. A common error is assuming independence and using $\frac{13}{52} \times \frac{13}{52}$, but this overlooks the deck change after the first draw. A helpful strategy is to read it as 'first a heart, then another heart given the first was removed.'

2

In a sequential game, a player must complete two actions. First, they land on a target (event $A$) with probability $P(A)=0.30$. Then, given they landed on the target, they score a bonus (event $B$) with probability $P(B\mid A)=0.60$. What is the probability that both events occur, $P(A\cap B)$?

$0.18$

$0.30+0.60=0.90$

$0.90$

$0.30$

Explanation

This question tests the Multiplication Rule for Probability, for finding the joint probability in a sequential game with dependent actions. The rule treats 'and' as conditional sequencing: first event A, then event B given A. Here, event A is landing on the target, with P(A) = 0.30. Given landing on the target, the conditional probability of scoring a bonus (event B) is P(B|A) = 0.60. Thus, P(A ∩ B) = 0.30 × 0.60 = 0.18, the probability of both landing and scoring. Commonly, people assume independence and add probabilities, but bonus scoring depends on landing. A good approach is to parse it as 'first land on target, then score given that landing.'

3

A student applies for a scholarship in two steps. First, their application is submitted on time (event $A$) with probability $P(A)=0.85$. Then, given it was submitted on time, it is accepted for review (event $B$) with probability $P(B\mid A)=0.40$. What is the probability that both events occur, $P(A\cap B)$?

$1.25$

$0.85+0.40=1.25$

$0.34$

$0.85$

Explanation

This question tests the Multiplication Rule for Probability, to find the joint probability in a two-step scholarship application process. The rule interprets 'and' as sequential and conditional: first event A, then event B given A. Event A is submitting on time, with P(A) = 0.85. Given on-time submission, the conditional probability of acceptance for review (event B) is P(B|A) = 0.40. Thus, P(A ∩ B) = 0.85 × 0.40 = 0.34, the probability of both timely submission and acceptance. People frequently mistake this for independent events and add probabilities, but review depends on timeliness. A strategy is to read it as 'first on time, then accepted given on time.'

4

A quality-control process checks items in two stages. First, an item is chosen from Batch 1 (event $A$) with probability $P(A)=0.25$. Then, given it is from Batch 1, it is defective (event $B$) with probability $P(B\mid A)=0.08$. What is the probability that both events occur, $P(A\cap B)$?

$0.08$

$0.25\cdot 0.25=0.0625$

$0.02$

$0.33$

Explanation

This question tests the Multiplication Rule for Probability, to compute the joint probability in a two-stage quality-control process. It frames 'and' as sequential: first event A occurs, then event B conditional on A. In this setup, event A is choosing from Batch 1, with P(A) = 0.25. Then, given Batch 1, the conditional probability of it being defective (event B) is P(B|A) = 0.08. The result is P(A ∩ B) = 0.25 × 0.08 = 0.02, the probability of both from Batch 1 and defective. A typical error is treating events as independent and squaring probabilities, disregarding the conditionality. To counter this, read it as 'first from Batch 1, then defective given that batch.'

5

A manufacturing line has a two-step outcome. First, an item is assembled by Robot 1 (event $A$) with probability $P(A)=0.55$. Then, given it was assembled by Robot 1, it is packaged correctly (event $B$) with probability $P(B\mid A)=0.92$. What is the probability that both events occur, $P(A\cap B)$?

$0.55+0.92=1.47$

$0.55\cdot 0.55=0.3025$

$0.506$

$0.92$

Explanation

This question tests the Multiplication Rule for Probability, computing the joint probability in a manufacturing line with dependent steps. It treats 'and' as a sequence: first event A, then event B conditional on A. Event A is assembly by Robot 1, with P(A) = 0.55. Given Robot 1 assembly, the conditional probability of correct packaging (event B) is P(B|A) = 0.92. Multiplying gives P(A ∩ B) = 0.55 × 0.92 = 0.506, the probability of both Robot 1 assembly and correct packaging. A usual error is assuming independence and squaring or adding, but packaging depends on the assembler. Parse as 'first assembled by Robot 1, then packaged correctly given that.'

6

A factory inspects products in two steps. First, a product is selected from the day’s production; the probability it is from Line A is $P(A)=0.40$. Then, given that it came from Line A, the probability it passes the final inspection is $P(B\mid A)=0.95$. What is the probability that both events occur (the product is from Line A and it passes final inspection), i.e., $P(A\cap B)$?

$1.35$

$0.40$

$0.38$

$0.95$

Explanation

This question tests the Multiplication Rule for Probability, which calculates the joint probability of two dependent events occurring together. The rule expresses P(A and B) as P(A) multiplied by P(B given A), interpreting 'and' as a sequential process: first event A occurs, then event B given that A has happened. In this factory inspection context, the first event A is selecting a product from Line A, with P(A) = 0.40. Then, given it is from Line A, the conditional probability of passing final inspection (event B) is P(B|A) = 0.95. Multiplying these gives P(A ∩ B) = 0.40 × 0.95 = 0.38, representing the probability that a product is from Line A and passes inspection. A common mistake is assuming independence and multiplying unrelated probabilities, which ignores the dependency on the production line. To avoid this, read the scenario as 'first the product is from Line A, then it passes given that origin.'

7

A two-step lab test works as follows. First, a sample is contaminated (event $A$) with probability $P(A)=0.10$. Then, given the sample is contaminated, the test returns positive (event $B$) with probability $P(B\mid A)=0.90$. What is the probability that both events occur, $P(A\cap B)$?

$0.10\cdot 0.10=0.01$

$0.10+0.90=1.00$

$0.90$

$0.09$

Explanation

This question tests the Multiplication Rule for Probability, calculating the joint probability in a two-step lab test with dependency. The rule sees 'and' as a sequence: first event A, then event B given A. Event A is the sample being contaminated, with P(A) = 0.10. Given contamination, the conditional probability of a positive test (event B) is P(B|A) = 0.90. Multiplying provides P(A ∩ B) = 0.10 × 0.90 = 0.09, the probability of both contamination and positive result. Often, independence is wrongly assumed, leading to adding or squaring, but positivity depends on contamination. Interpret as 'first contaminated, then positive given contamination' for accuracy.

8

A jar contains 5 red marbles and 3 blue marbles. Two marbles are drawn without replacement. Let event $A$ be “the first marble is red,” and event $B$ be “the second marble is blue.” Given $P(A)=\frac{5}{8}$ and $P(B\mid A)=\frac{3}{7}$, what is the probability that both events occur, $P(A\cap B)$?

$\frac{5}{8}\cdot\frac{3}{8}=\frac{15}{64}$

$\frac{5}{8}+\frac{3}{7}$

$\frac{15}{56}$

$\frac{3}{7}$

Explanation

This question tests the Multiplication Rule for Probability, applied to dependent draws without replacement from a jar of marbles. It interprets 'and' as a sequence: first event A, then event B conditional on A. Event A is drawing a red marble first, with P(A) = 5/8 from 5 red out of 8 total. Given a red was drawn, the conditional probability of then drawing a blue (event B) is P(B|A) = 3/7, with 3 blue left out of 7 marbles. Multiplying gives P(A ∩ B) = (5/8) × (3/7) = 15/56, the probability of first red then blue without replacement. A frequent mistake is assuming independence and using original fractions for both, ignoring the updated counts. Read it as 'first red, then blue given one red is gone' to apply the rule correctly.

9

A website login process has two sequential checks. First, a user enters a correct username (event $A$), which happens with probability $P(A)=0.90$. Then, given the username is correct, the user enters the correct password (event $B$) with probability $P(B\mid A)=0.80$. What is the probability that both events occur, $P(A\cap B)$?

$1.70$

$0.80$

$0.90+0.80=1.70$

$0.72$

Explanation

This question tests the Multiplication Rule for Probability, which finds the joint probability for sequential, dependent events in a login process. The rule views 'and' as conditional: first event A succeeds, then event B given A. In this website login, event A is entering the correct username, with P(A) = 0.90. Then, given a correct username, the conditional probability of entering the correct password (event B) is P(B|A) = 0.80. The product is P(A ∩ B) = 0.90 × 0.80 = 0.72, the probability of both steps succeeding. People often mistakenly assume independence and add probabilities, but here password entry depends on username success. To prevent this, think of it as 'first correct username, then correct password given that.'

10

A bag contains 6 black socks and 4 white socks. Two socks are drawn without replacement. Let event $A$ be “the first sock is white,” and event $B$ be “the second sock is white.” Given $P(A)=\frac{4}{10}$ and $P(B\mid A)=\frac{3}{9}$, what is the probability that both events occur, $P(A\cap B)$?

$\frac{4}{10}$

$\frac{4}{10}\cdot\frac{3}{9}=\frac{2}{15}$

$\frac{4}{10}+\frac{3}{9}$

$\frac{2}{5}\cdot\frac{2}{5}=\frac{4}{25}$

Explanation

This question tests the Multiplication Rule for Probability, for dependent sock draws without replacement from a bag. It views 'and' as conditional: first event A, then event B given A. Event A is drawing a white sock first, with P(A) = 4/10 from 4 white out of 10 total. Given the first was white, the conditional probability of another white (event B) is P(B|A) = 3/9, with 3 white left out of 9. The product is P(A ∩ B) = (4/10) × (3/9) = 2/15, the probability of two white socks in sequence. A common pitfall is assuming independence and using (4/10) twice, ignoring the reduced count. To avoid errors, think 'first white, then white given one is removed.'

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