This is like calculating a grade in class that uses weighted categories.
Imagine you have a class where your grade will be determined by the following criteria.
Homework = 20%
Labs = 30%
Tests = 50%
If you have 100% on your homework, 90% on your labs and 80% on your tests your grade will not average to 90%!! It will actually average out to be 87%.
100 x 0.2 = 20
90 x 0.3 = 27
50 x .8 = 40
I multiplied each number by the decimal form of the percentage (20% comes from 0.2 etc.)
Average = 20 + 27 + 40 = 87
This is due to the fact that your tests are much more important for your grade than your homework grades are based on the criteria used by your teacher.
Here is an example looking at the element magnesium.
Hope this helps