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The amount of jen's monthly phone bill is normally distributed with a mean of $76 and a standard deviation of $9. what percent of her phone bills are between $49 and $103

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barnuts

To solve this problem, we make use of the z statistic. The formula for z score is:

z = (x – u) / s

where x is sample value, u is the mean and s is the standard deviation

 

z (x = 49) = (49 – 76) / 9 = -3

Using the standard tables, P (z = -3) = 0.0013

 

z (x = 103) = (103 – 76) / 9 = 3

Using the standard tables, P (z = 3) = 0.9987

 

Hence,

P (-3 <= z <=3) = 0.9987 – 0.0013 = 0.9974

 

Therefore 99.74%

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