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## #1 2014-04-01 17:33:15

iLloyd054
Member
Registered: 2014-04-01
Posts: 10

### Probability Problem

I got two questions and its tricky for me to tackle them, need help!

Question 2

a)In a test of Shettles method to increase the probability of conceiving a boy. Among 574 women using that method, 525 had baby boys. Assuming that the method has no effect so that boys and girls are equally likely, find the probability of getting at least 525 boys among 574 babies. Justify the types of distribution used in this experiment. Does the result suggest that the Shettles method is effective? Explain.

b)If you buy a lottery ticket in 50 lotteries, in each of which your chance of winning a price is 1/100,
i)what is the probability that you will win a prize at least one ?
ii)what is the probability that you will win a prize exactly once ?
iii) what is the least number of lotteries that you should go so that the probability of win a prize at least one is at least 0.5 ?

c)In a large class, suppose your instructor tells you that you need to obtain a grade in the top 10% of your class to get an A on a particular exam. From past experience she is able to estimate the mean and standard deviation on this exam will be 72 and 13, respectively. Assume that the grades will be approximately normal distributed,
i)    What will be the minimum grade needed to obtain an A?
ii)    Find the 33rd percentile for exam scores.
iii)What is the probability that a person picked at random has an exam score of 60?

Last edited by iLloyd054 (2014-04-01 22:54:38)

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## #2 2014-04-01 19:45:30

bobbym
From: Bumpkinland
Registered: 2009-04-12
Posts: 90,663

### Re: Probability Problem

Hi;

For b) ii)

For b) i)

For b) iii) You have not copied the question exactly as it appears in the book. I will solve it making some assumptions. Please be more precise when asking a question.

The answer is you will need at least 69 to have a 50% or better chance of winning 1 or more prizes.

c)i) I am getting 89.

In mathematics, you don't understand things. You just get used to them.

I agree with you regarding the satisfaction and importance of actually computing some numbers. I can't tell you how often I see time and money wasted because someone didn't bother to run the numbers.

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## #3 2014-04-02 21:57:08

iLloyd054
Member
Registered: 2014-04-01
Posts: 10

### Re: Probability Problem

thanks bobbym  for the b)i) and b)ii) i manage to get same answers like you, yey!.

The question is ok I double check it.

c) am not sure but i think we are using Normal distribution.
and please can you show me calculation part, if ts ok with you, coz am not really god at this, maybe I can learn something new from you.thanks

Last edited by iLloyd054 (2014-04-02 22:16:02)

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## #4 2014-04-03 04:27:27

bobbym
From: Bumpkinland
Registered: 2009-04-12
Posts: 90,663

### Re: Probability Problem

Hi;

For c) i) I do not make use of tables very much anymore so I do the numerical integrations instead.

You can see that a grade of 88 is not quite enough and that a grade of 89 is required.

If you can not do the integrations then you will have to use z scores and tables.

In mathematics, you don't understand things. You just get used to them.

I agree with you regarding the satisfaction and importance of actually computing some numbers. I can't tell you how often I see time and money wasted because someone didn't bother to run the numbers.

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