The length of time required to to complete a college achieve

The length of time required to to complete a college achievement test is found to be normally distributed with mean 70 minutes and standard deviation 12 minutes. When should the test be terminated if we wish to allow sufficient time for 90% of the students to complete the tests?

Solution

First, we get the z score from the given left tailed area. As          
          
Left tailed area =    0.9      
          
Then, using table or technology,          
          
z =    1.281551566      
          
As x = u + z * s,          
          
where          
          
u = mean =    70      
z = the critical z score =    1.281551566      
s = standard deviation =    12      
          
Then          
          
x = critical value =    85.37861879   [ANSWER]