MATHS PROBABILITY

kitty

2010-01-29 5:10 am

1)out of 1200 families with 4 children each.how many families would be xpected to have at least one boy?
2)A student estimates that his chance of passing english,maths and chinese are 0.9,0.8 and0.75 respectively.Find the probability that a)he passes english only.b)he passes english and maths only.c)he passes all three.d)he fails in each subject.

回答 (1)

用戶9112

2010-01-29 5:20 am

✔ 最佳答案

(1) Out of 4 children, the probability of having no boy is (1/2)(1/2)(1/2)(1/2)
= 1/16
The probability of having at least one boy = 1 – 1/16 = 15/16
The expected number of families = 1200 * 15/16 = 1125
(2) (a) Probability of passing English only = (0.9)(1 – 0.8)(1 – 0.75)
= 0.045
(b) Probability of passing English and Maths only = (0.9)(0.8)(1 – 0.75)
= 0.18
(c) Probability of passing all three = (0.9)(0.8)(0.75) = 0.54
(d) Probability of failing all = (1 – 0.9)(1 – 0.8)(1 – 0.75)
= 0.005

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