A bag contains 3 red cubes, 4 yellow cubes, and 3 green cubes. You choose a cube without looking, note its color, and put the cu
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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