The number of cell phone minutes used by high school seniors

The number of cell phone minutes used by high school seniors follows a normal distribution with a mean of 500 and a standard deviation of 50. What is the probability that a student uses more than 580 minutes?

Solution

We first get the z score for the critical value. As z = (x - u) / s, then as          
          
x = critical value =    580      
u = mean =    500      
          
s = standard deviation =    50      
          
Thus,          
          
z = (x - u) / s =    1.6      
          
Thus, using a table/technology, the right tailed area of this is          
          
P(z >   1.6   ) =    0.054799292 [ANSWER]