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 

.
 
        
        
        
Answer:
5 times as many should be your answer.
 
        
                    
             
        
        
        
Answer:
Step-by-step explanation:
b - 7x = a - b....subtract b from both sides
-7x = a - b - b....combine like terms
-7x = a - 2b....divide both sides by -7
x = (a - 2b) / -7....or it could be written as x = -1/7a + 2/7b