When the weights are not in percent hours or points you should also divided by the sum of the weights. If the grade is above 300 grade is C. Letter Grades and Grade Points.
But you want to know what you need to get on the coming test in order to get an overall average of 88.
In general you calculate the mean or average of a set of numbers by adding them all up and dividing by how many numbers you have. If you want to guess the grade you will end with in the class you will need to ask your teacher for the total number of points that can be earned in the class. Multiple each grade by its weight. IF the grade is not matching any of the above conditions then the grade will be FAIL.