# Finding probability with given mean and standard deviation

#### k1mpossible

##### New Member
Hi, I have tried to do this using the z value method but I'm not even sure if I'm right.
The question is:
"The recommended total blood cholesterol for a lower risk of heart disease is less than 200mg/dL. The total blood cholesterol for males aged 20 -29 is normally distributed with a mean of 195 and a standard deviation of 9.2. What is the probablity that a male aged 20 - 29 chosen at random has a total blood cholestoral greater than 200? What is the probability that a male aged 20 - 29 have a blood cholestoral in the range 180 - 200?

My answer for the first part is 0.2981

If someone could just explain to me and point me in the right direction, that would be heaps of help. Thank you.

#### snorelax

##### New Member
That first number seems correct to me. To find your answer, I assumed you found the area to the right of the corresponding z-score or did 1-(area left of z-score). To perform the second part, simply find the area to the left of z-score of 200 and subtract from the area left of z score of 180.

#### k1mpossible

##### New Member
oh, so my first answer is correct?
I actually typed the second part (second question) wrong, it asked for proportion not probability. I don't know what that means.

Can you please explain to me?

#### k1mpossible

##### New Member
That first number seems correct to me. To find your answer, I assumed you found the area to the right of the corresponding z-score or did 1-(area left of z-score). To perform the second part, simply find the area to the left of z-score of 200 and subtract from the area left of z score of 180.
Yep, that's what I did using the z table to find the percentage.
The second question actually asked for the PROPORTION, not the probability, but I don't know what that means.

It'll be great if you could help me.
Thanks

#### snorelax

##### New Member
well if im understanding this question correctly, the proportion should be equal to the probability. usually proportion should make you automatically think a fraction. the numerator should be the part that you want and the denominator should be the whole of the field that youre working with. So in this case, the probability of z(200-180) divided by the entire population of the z table which is just 1. But say it asked you what proportion of people with a blood cholesterol of over 200 had a blood cholesterol of over 250. You would then have the fraction of the area of the 250 over the area of 200(your first answer). This, in essence, is a bayes problem in case you're wondering.