You park in a parking garage 3 floors below ground level. Then you get in an elevator and go up 9 floors. How many floors are yo
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Let 
denote the number of minutes spent brushing his teeth, and let 
be the mean and 
the standard deviation for this distribution.
The z-score corresponding to this probability is approximately
, which means
Next, (note the sign change)
The corresponding z-score is approximately
, so you have
Solving the two equations for and , you'll find that the mean is approximately 
and the standard deviation is approximately 
.
The z-score corresponding to this probability is approximately
Next, (note the sign change)
The corresponding z-score is approximately
Solving the two equations for