# Thread: Normal Probability and finding standard deviation

1. ## Normal Probability and finding standard deviation

"The weekly earnings of fastfood restaurant employees are normally distributed with a mean of $395. If only 1.1% of the employees have a weekly income of more than$429.35, what is the value of the standard deviation of the weekly earnings of the employees?"

Can someone not just provide and answer, but HOW to come to this answer? I am confused. I can't even find this type of question answered in my college book. Calculations would be great.

Thanks,

Lorelei7

2. ## Re: Normal Probability and finding standard deviation

Does your book have a section on Normal Distributions?

You need to find the Z-score from a table like this:

http://www.beanactuary.org/exams/pre...ion_Table.html

Find the Z-score for 1 - 0.011 = .989

Then solve

3. ## The Following User Says Thank You to asterisk For This Useful Post:

Lorelei7 (10-22-2013)

4. ## Re: Normal Probability and finding standard deviation

If you knew the mean and standard deviation would you be able to find the proportion of employees that have a weekly income of more than 429.35?

5. ## Re: Normal Probability and finding standard deviation

so;
0989=429.35-395/stand dev
.989 = 34.35/stand dev then multiply stand dev on both sides = .989(std dv) = 34.35
.989/.989 (to get std. dv alone) = 34.35/.989 = std dev=34.7321 ?
Is this correct? From this I am still unsure if the 34.7321 is correct?

Please forgive me as IDH the sigma sign handy; as well as being confused if this is the answer?

6. ## Re: Normal Probability and finding standard deviation

Sorry but no. .989 is not the Z-score. You need to look up .989 in the Normal Distribution Table and find the Z-score. Then use that number in the equation

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