Normal distribution problem

Hi, I'm trying to finish my homework and I'm stuck on a problem. Here it is: Suppose that the ages at a time of onset of a certain disease are normally distributed with a mean of 11.5 years and a standard deviation of 3 years.

In a population of 10,000 with the disease, how many would you expect to be between the ages of 7.5 and 13.5 years?

So here is what I did:

z=13.5-11.5 / 3 = 0.67

z=7.5-11.5 / 3 = -1.33

I look up those numbers in my book for the standard normal distribution and I get:

0.67 = .7486

-1.33 = .0918

So then I added .7486 and .0918 which gave me .8404 then I multiplied by 10,000 so I get (10,000)(.8404)=8,404 people between 7.5 and 13.5 have the disease. Did I do this right?