the length of pregnancies of humans are normally distributed

Solution

We first get the z score for the critical value. As z = (x - u) / s, then as          
          
x = critical value =    250      
u = mean =    268      
          
s = standard deviation =    15      
          
Thus,          
          
z = (x - u) / s =    -1.2      
          
Thus, using a table/technology, the left tailed area of this is          
          
P(z <   -1.2   ) =    0.11506967 [ANSWER]