Answer: the answer is 0.5
Step-by-step explanation:
Y=mx+b mx= slope b= y intercept
Find the z-scores for the two scores in the given interval.

For the score x =391,

.
For the score x = 486,

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%.
Here's the equation,please do ask if you don't understand. Sorry for my terrible hand writing ^^; hope it helps!