bio statistics

Total Cholesterol in children aged 10 to 15 years of age is assumed t follow a normal distribution with a mean of 191 and a standard deviation of 22.4.
a) what proportion of children 10 to 15 years of age have total cholesterol between 180-190?
b) what proportion of children 10 to 15 years of age would be classified as hyperlipidemic (assume that hyperlipidemia is defined as a total cholesterol level over 200)?
c) what is the 90th percentile of total cholesterol?

Ok, so that is the question.

I understand that there are 6 study persons..10,11,12,13,14,& 15 year old's. I plugged in the mean given and the standard deviation that was given...what I don't understand is how do I get the other information...I got the cholesterol information from the first question but am not sure if it correct. If anybody is able to help me with this problem I would greatly appreciate it. I took this class as a filler, it isn't need for my master's degree but I am not able to drop it at this time.

x Cholesterol Mean standard deviation probability
10 180 191 22.4 11
11 185 191 22.4 6
12 190 191 22.4 1
13 195 191 22.4 -4
14 200 191 22.4 -9
15 200+ 191 22.4 not sure what this number would be or if the other numbers are correct.

b) looks like it might be the easiest.

Calculate the z-score

(200-191)/22.4 = 9/22.4 = 0.402

A VERY ROUGH empirical rule might suggest that we should be somewhere in the neighborhood of 0.50 - 0.4*0.34 = 0.50 - 0.14 = 0.36

A closer look at a table or calculator (using total values of the cummulative density function) provides 1.00 - 0.66 = 0.34

The emperical rule wasn't too far off.

Part a) is very similar. It's just in 2 pieces. Let's see what you get. Part c) is backwards, but is the same principle.