Question

The heights of 20-29 year old males in the US are approximately normal, with mean 70.4 inches and standard deviation 3.0 inches.

1) Find the height of 20 to 29 year old males that falls at the 90th percentile?

Answer 1

In other words, you want to find the z value (standard score) for which the area to the LEFT of this z-value is 0.90. Once y ou have that , you can calculate the height that corresponds to that. done that before?

Answer 2

So do I look at the z-value on the table that is that score? Which would be 0.8159?

Answer 3

Hold; I'm checking on my calculator.

Answer 4

I found that on the z score table

Answer 5

According to my calculation, the z-score corresponding to an area of 0.9 to the left is 1.282. Double check what you've done.

Answer 6

I'm not sure exactly how to do this problem, what equation am I using and how do I plug the information in?

Answer 7

Do I used (k/100)(n) ?

Answer 8

You have a table of z scores. Look in the body of that table for the area / probability 0.9 (or whatever value is closest to 0.9000) In the leftmost column, find the corresponding z-score. Again, I believe z-1.282. Try it.

Answer 9

You could also do this calculation with a TI-84 calculator, if you have one; the function to use is invNorm(.

Answer 10

I have a TI 34 calculator. The Z score (if I'm remembering to use the table correctly) since it's positive of 0.9000 is look down the left column for 0.90 and then over one for the 0.00 which gets me 0.8159. The other way around gets me 0.5359, and that's strictly off the table without calculations.

Answer 11

I don't have anything going over 0.9999

Answer 12

No one else does either!! :)

K: Actually you look for 0.9000 in the BODY (not left column) of the table, and once you've found it, you find the 1st 2 digits of the z-score in the left column. Try that.
I may be delayed a few min in replying to your next comment; don't worry; I'll be back.

Answer 13

Oooh okay so the closest numbers to 0.9000 is 0.8997 and the corresponding numbers are 1.2 and 0.08 which would give me 1.280. Where do I go from there then?

Answer 14

z = (x - mean)/standard deviation

Answer 15

So (1.280-70.4) /3.0?

Answer 16

no, you're supposed to find x. 1.28 is z

Answer 17

ok ok let me try that

Answer 18

I got 74.24, that seems correct

Answer 19

yes

Answer 20

kaldrid: Your "Oooh okay so the closest numbers to 0.9000 is 0.8997 and the corresponding numbers are 1.2 and 0.08 which would give me 1.280. Where do I go from there then?" is exactly what I was hooping you'd find.
X - Mean
How is the z score defined? z = ----------
std, dev

In this case you know z, the mean and the std. dev but must calculate x.

sourwing was correct when he typed the following.
z = (x - mean)/standard deviation

Please try now to find x.

Answer 21

The answer I got was 74.24 (inches)

Answer 22

THank you all! You've all been wonderful :)

Answer 23

My great pleasure! See you again.