some maths questions...(急15點.)

2010-11-02 9:05 pm

1)The scores of a Mathematics test taken by 400 students are normally distributed with a mean of 60 and standard deviation of 10. The passing mark of the test is 40.
i)How many students scored more than 80?
ii)What is the probability that a randomly chosen student failed in the test?

2)It is known that 10% of the appliances produced by a production plant are defective. Five of those appliances are going through QC inspections.
i)What is the probability that 4 out of the 5 appliances are defective?
ii)What is the probability that at lease one of the 5 appliances is/are defective?

回答 (1)

myisland8132

2010-11-02 9:16 pm

✔ 最佳答案

1i) Since 80 is 2 standard deviation away from the mean P(X>80)=P(Z>2)=0.02275. The students scored more than 80 = 400*0.02275=9

ii) P(X<40)=P(Z<2)=0.02275

2i) (5C4)(0.1)^4(0.9)=0.00045
ii) 1-(0.9)^5=0.40951

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