Originally Posted by

**DryMouse**
Here is how I did it, let's see if I am right:

First, I made a Standard Graph showing that we want the bottom 10% shading to the left.

Then, let (mean) u=100, (Standard Diviation) o=15 and we find for P10 (which I am calling x)

Next, if you are working in a Stats book, it should have some sort of Z-Score table which you can look up what the Zscore of 10% is, in this case -1.28, then plug it into the formula: x=u+(z*o)

We get x=100+(-1.28*15); x=80.8

Therefore; x=81 (rounded) I.Q. score separates the bottom 10% from the top 90%.

Someone correct me if I am wrong.