DP Math AI · HL / SL · Statistics and Probability

SL 4.8—Binomial distribution

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  1. Question 1

    A factory produces bolts, with 8% being defective. A random sample of 25 bolts is selected. Let X be the number of defective bolts. Which distribution correctly models X?
    No clue? Show me the answer
    Correct answerCorrect!Incorrect
    AX∼B(25,0.08)

    Step-by-step walkthrough

    Choose a solution method

    Method #1Worked solution

    Step 1: Identify n and p

    The number of trials is n=25 bolts, and the probability of success (defective) is p=0.08.

    Step 2: Apply binomial notation

    Since trials are fixed, independent, with two outcomes and constant probability, X∼B(n,p) with n=25, p=0.08.

    Step 3: Write distribution

    This gives X∼B(25,0.08).

    Step 4: Select answer

    The correct notation is X∼B(25,0.08), option A.

    Method #2Why the others are wrong

    Step 1: Option B

    Swaps n and p, giving 8 trials with probability 0.25 - incorrect order of parameters.

    Step 2: Option C

    Uses the probability of failure (0.92) instead of success (0.08) as p.

    Step 3: Option D

    Reverses the order of parameters entirely, writing p before n, which is not standard notation.

    Step 4: Conclusion

    Only option A correctly assigns n=25 and p=0.08 in the correct order.

  2. Question 2

    Which of the following scenarios satisfies all four conditions required for a binomial distribution?
    No clue? Show me the answer
    Correct answerCorrect!Incorrect
    BRolling a fair 6-sided die 12 times and counting the number of 4s rolled

    Step-by-step walkthrough

    Choose a solution method

    Method #1Worked solution

    Step 1: Check each condition

    A binomial setup needs fixed trials, two outcomes, constant probability, and independence.

    Step 2: Test option B

    Rolling a die 12 times: n=12 is fixed, success = 'rolled a 4', p=1/6 constant, rolls are independent.

    Step 3: Confirm all conditions met

    All four conditions hold, so this is a valid binomial scenario.

    Step 4: Select answer

    Option B is the correct binomial scenario.

    Method #2Why the others are wrong

    Step 1: Option A

    Sampling without replacement changes the probability of drawing a spade each time, violating constant probability.

    Step 2: Option C

    There is no fixed number of trials; the number of days is variable, not fixed.

    Step 3: Option D

    This is a 'trials until first success' scenario with no fixed n, which follows a geometric distribution, not binomial.

    Step 4: Conclusion

    Only option B has a fixed n, two outcomes, constant p, and independence.

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← Previous topicSL 4.7—Discrete random variablesNext topic →SL 4.9—Normal distribution
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