ITPEC IP October 2021 Question 1
ITPEC IP October 2021 — Question 1 of 100
Picking black, black, black has the lowest probability. Without replacement, its probability is 4/7 × 3/6 × 2/5 = 24/210. Each sequence containing exactly two black balls and one white ball has probability 4 × 3 × 3 divided by 7 × 6 × 5 = 36/210; the order changes the factors but not their product.
Answer (a)
Why not others:
- black-black-white, black-white-black, and white-black-black each contain the same counts and therefore have the same probability, 36/210
Key rule: For sampling without replacement, multiply the changing conditional probabilities at each draw.
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