Find the z-scores for the two scores in the given interval.
[tex]z=\frac{x-\mu}{\sigma}[/tex]
For the score x =391, [tex]z=\frac{391-486}{95}=\frac{-95}{95}=-1[/tex].
For the score x = 486, [tex]z=\frac{486-486}{95}=0[/tex]
Now you want the area (proportion of data) under the normal distribution from z = -1 to z = 0. The Empirical Rule says that 68% of the data falls between z = -1 to z = 1. But the curve is symmetrical around the vertical axis at z = 0, so the answer you want is HALF of 68%.
