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標題:

Probability ..... plx help

發問:

There are 6 MC questions in a short quiz and 5 options are given for each question 2 marks are awarded for a correct answer and 1 mark is deducted for a wrong answer. If a student answers the questions by guessing find the probability that A) His total score is negative B) His total score is positive

最佳解答:

If he answers 1 questions correctly, he can answer 2 more question wrongly that his score is 0. Therefore, P(positive) = P(answering at least 3 questions correctly) = 6C3 x (1/5)(1/5)(1/5)(4/5)(4/5)(4/5) + 6C4 x (1/5)4 (4/5)2 + 6C5 x (1/5)5 (4/5) + (1/5)6 = 309 / 3125 P (0 marks) = P(answering exactly 2 questions correctly) = 6C2 x (1/5)2 (4/5)4 = 768 / 3125 P(negative) = 1 - P(positive) - P(0 marks) = 2048 / 3125

其他解答:

A. P(Negative Score) = P(1 Question is correct) + P(0 Question is correct) P(1 correct) = (6 choose 1) (1/5) P(0 correct) = 4/5*4/5*4/5*4/5*4/5*4/5 B. P(Positive Score) = 1 - P(Neagative) - P(Getting a score of zero which is 2Question is correct) P(2 correct) = (6 choose 2) (1/5) => The rest u do it urself baby.A9A3995996B435E3