*****Statistics Sum ***** ( Statistics Sum )

1)      If light bulbs have lives that are normally distributed with a mean of 2500 hours and a standard deviation of 500 hours, what percentage of light bulbs have a life less than 2500 hours?

 

2)The lifetimes of light bulbs of a particular type are normally distributed with a mean of 370 hours and a standard deviation of 5 hours. What percentage of bulbs has lifetimes that lie within 1 standard deviation of the mean on either side? 

 

3)      The amount of Jen’s monthly phone bill is normally distributed with a mean of $60 and a standard deviation of $12.  Fill in the blanks:

68% of her phone bills are between $______________ and $______________.

 

4)The amount of Jen’s monthly phone bill is normally distributed with a mean of $50 and a standard deviation of $10.  Find the 25th percentile.

 

5) The diameters of bolts produced by a certain machine are normally distributed with a mean of 0.30 inches and a standard deviation of 0.01 inches.  What percentage of bolts will have a diameter greater than 0.32 inches?

 

6)The annual precipitation amounts in a certain mountain range are normally distributed with a mean of 88 inches, and a standard deviation of 10 inches. What is the likelihood that the mean annual precipitation during 25 randomly picked years will be less than 90.8 inches?  

 

7)  A final exam in Statistics has a mean of 73 with a standard deviation of 7.73.  Assume that a random sample of 24 students is selected and the mean test score of the sample is computed. What percentage of sample means are less than 70?  

 

8)   A mean score on a standardized test is 50 with a standard deviation of 10. Answer the following

1.     
What scores fall between –1 and +1 standard deviation?

2.     
What percent of all scores fall between –1 and +1 standard deviation?

3.     
What score falls at +2 standard deviations?

 

4.     
What percentage of scores falls between +1 and +2 standard deviations?