An exercise contains 9 questions. A teacher assigns 3 questions to students.
A student ignores the teacher's instruction and answer 5 questions randomly. Find the probability that he will answer
a) all 3 questions assigned by the teacher
b) only one question assigned by the teacher
Please help! Thank you!
F.4 Mathematics Probability(1)
2014-06-15 8:58 am
回答 (1)
2014-06-15 10:28 am
✔ 最佳答案
IdeaSeparate the 9 questions into 2 groups:
(1) One group of 3 questions which are assigned by the teacher;
(2) One group of 6 questions which are not assigned by the teacher.
Solution
(a) The required probability is
= ( ₃C₃ × ₆C₂ ) / ₉C₅ [Notice that 3 + 6 = 9, 3 + 2 = 5]
= ( 1 × 15 ) / 126
= 5/42
(b) The required probability is
= ( ₃C₁ × ₆C₄ ) / ₉C₅ [Notice that 3 + 6 = 9, 1 + 4 = 5]
= ( 3 × 15 ) / 126
= 5/14
收錄日期: 2021-04-15 15:46:02
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https://hk.answers.yahoo.com/question/index?qid=20140615000051KK00013