I need to calculate the score of a student's grade in a class. The class has five assignments, M1, M2, PA1, PA2, and Final. Only four assignments count towards the grade, M1, M2, PA1, and PA2. These four assignments are weighted differently towards the grade in the class, which is "ScoreA" in the picture below.

http://i42.tinypic.com/20h5504.png

Here is how the assignments are weighted toward the grade:

M1 - 20% of ScoreA
M2 - 30% of ScoreA
PA1 - 25% of ScoreA
PA2 - 25% of ScoreA

ScoreA for the first student (95002112) in the picture is 86.25. I need to come up with a formula to calculate that ScoreA.

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This is part of my homework assignment, so I'm not asking you to give me the answer. I have very little experience in Excel, so can someone help me a little, please. I understand the math, it's just that I don't understand how to come up with one formula for this. Please if you can help, that would be awesome. Thank you.