Help with 2 diff types of problems

[BillBob]

If standard deviation is 80 and mean is 10:

P ( X < 100)

How do you do that?

...

And, confidence intervals are very hard. Can someone break how to do a 95% interval down. Say 1000 people were interviewed about smoking and 600 said they did. How would you do that interval?

 

[royhaas]

For the first problem, convert to a z-score and use the standard normal distribution table in your textbook. For the second, read your text on how to compute a confidence interval for a proportion.

 

[BillBob]

Thanks for the tips. How would you convert that to a z-score? The last third of this course has me confused.

 

[pka]

Subtract the mean and divide by the standard deviation.
So you get \(\displaystyle P\left( {z < \frac{9}{8}} \right).\)