I'm an undergraduate student trying to create a formula so that whenever I put in my alphabet semester's exams grades, those letters will be "converted" to as number values to make calculations possible (please see the attachment for the explanations).
______________________________

I know I could directly input the grades as values but, rather frequently, me and my friends end up inputting wrong grade values (and sometimes the whole calculations). I just wanted to easily input the letters, and let Excel will calculate it for me.

I don't think "VLOOKUP" or "COUNTIF" will be of help since I will keep dealing with different numbers everytime, though I could be well wrong; I'm no expert :(