You can put this solution on YOUR website!
We need to find the SD that yields 90% below it. There are tables of SD that give area between +-SD. Let's use that type of table.
In order to find the SD that gives 90% below, we need to allow for the fact that we are going to include the entire lower half of the normal curve. If we want the upper limit to include 90%, then we will have 40% above the mean.
We need to find the SD value from our table that yields 2*40% = 80% between +-SD. In my table, that is a value of 1.2816.
You can get this from your TI-83 like tis -->http://people.hsc.edu/faculty-staff/robbk/Math121/TI-83/InvStdNormal.html
So Jen is