What is 90th percentile out of 40?

For a complete definition of Percentile see the related link. A percentile is the value of a variable below which a certain percent of observations fall

The 90th percentile is typically the point at which 10 percent of the highest scores fall.

If you assume that scores are out of 40, and that they are evenly distributed (an unlikely event in real life), then the 90th percentile is all scores above 36.