Answer:

Step-by-step explanation:
Given

Putting x = -3 to find f(-3)

as
∵ 
so





∵ 
Thus,

It is 102.5. You made it kind of confusing at first by putting the divisor first in your question then putting your dividend, but I understand what you meant.
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

.