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## #1 2013-10-03 23:04:52

puccinitang
Guest

### Probability

Help me plz, thanks

A teacher claims that the time (in minutes) required for any HKU student to finish the homework is a normally distributed random variable with a mean of 30 minutes and a standard deviation of 10 minutes.

(i) If a student is randomly selected from HKU, what is the probability that he/she will use more than 27 minutes but less than 32 minutes to finish the homework?

(ii) If a sample of 6 students is randomly selected from HKU, find the probability that the average time of 6 students to finish the homework will be more than 32 minutes?

(iii) Suppose the time (in minutes) required to finish the homework is not a normal random variable. Under what circumstances can part (b) still be evaluated?

## #2 2013-10-03 23:09:03

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: Probability

Hi;

i) P(27 < X <= 32) = .1972

Under what circumstances can part (b) still be evaluated?

Where is part b?

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #3 2013-10-05 16:53:07

puccinitang
Guest

### Re: Probability

sorry, part b= part ii

## #4 2013-10-05 21:11:24

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: Probability

Hi;

ii) .3121

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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puccinitang
Guest

## #6 2013-10-06 10:19:25

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: Probability

Hi;

iii) The question is way too general and requires making too many assumptions. If I draw at random from a distribution of any shape and I draw a big sample then the distribution of the sample will approach a normal distribution. They usually pick n = 30.

If I draw a sample of 6 students from this unknown distribution then I will have great difficulty with ii)

The general answer for iii) is that I can get the answer if I know the distribution or if the random sample is 30 or more. This is the best I can do.

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #7 2013-10-07 06:27:53

puccinitang
Guest

### Re: Probability

thank you very much

## #8 2013-10-07 06:34:52

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: Probability

Hi;

Hope that was okay and welcome to the forum.

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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