normal distribution and standard deviation (help)

Question

It is claimed that the IQ scores of adult humans form a Normal distribution with a mean of 100 and a standard deviation of 15.
 
If I choose a person completely at random and administer an IQ test, what is the probabilty that the score lies between 91 and 130? (Give your answer to 4 decimal places.)

Answer 1

Obtain a z-score chart

http://www.stat.ufl.edu/~athienit/Tables/Ztable.pdf

Low score

z = (91 - 100) / 15

z = -9 / 15

z = -0.6

Percent below is 27.43%

High score

z = (130 - 110) / 15

z = 2

Percent below is 97.72%

 

What is the difference?