The graphs of y =1/x and y = 3/x – 4 can be compared by saying that c<span>ompared to the graph of y =1/x , the graph of y =3/x – 4 is a vertical stretch by a factor of 3 and a translation of 4 units down. This can be seen clearly when you graph the functions on a x-y coordinate plane.</span>
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

.