Light bulbs of a particular style have a mean life of 1600 h

Light bulbs of a particular style have a mean life of 1600 hours with a standard deviation of 200. what is the probability that one bulb will last over 1500 hours?

Solution

We first get the z score for the critical value. As z = (x - u) / s, then as          
          
x = critical value =    1500      
u = mean =    1600      
          
s = standard deviation =    200      
          
Thus,          
          
z = (x - u) / s =    -0.5      
          
Thus, using a table/technology, the right tailed area of this is          
          
P(z >   -0.5   ) =    0.691462461 [ANSWER]