If you want to be in the top 10%, then look up the z-score that corresponds to .9 in the z table. Then, the area to the right of that is the top 10%.

Look at the graph of the normal distribution. The z scores appraoch from the left.

That z score for .90 is approx. 1.28.

You need to earn at least 753.6 in order to be in the top 10%.

If you wanted to be in the top 5%, look up the z-score for .95.

Then, we would have

See now?.