The board of examiners that administers the real estate broker's examination in a certain state found that the scores are approximately normally distributed, the mean score on the test was 579, and the standard deviation was 72. Determine the 80th percentile of the scores.

Not sure what I did wrong?

My answer was:

80^th= .842 x=u+Zo 579+(.842)(72)=639.624

My teachers answer was 640 23.