Anormal curve is about the mean. consequently, 50% of the total area under a normal distribution curve lies on the left side of the mean, and 50% lies on the right side of the mean.

In statistics, when you gather data points for the same test, you construct a distribution graph. For example, you make a statistics based on the score of a class for a 145-item test. You graph it based on frequency such as, 10 students got 50 points and so and so forth. When that test have a few outliers and have more or less the same score, you form a normal distribution graph. It is more commonly known as the Gaussian Bell Curve. An example would be shown in the picture.

A normal distribution curve is symmetric about the mean. As a result, the area under the curve on the left side of the mean would be equal to the area under the curve on the right side of the mean. So, each would constitute 50%.