Look up the z score for the top 18% in a z table. In other words, look up .82 in a z table and find its corresponding z score.
Set the formula \(\displaystyle \frac{(x-\mu)\sqrt{n}}{\sigma}\) equal to this z score, enter in your
knowns, and solve for \(\displaystyle \mu\)
You have \(\displaystyle x, \;\ \sigma, \;\ n\)
