i think you are looking to keep Letter grades based on percentage of marks.

try this links

http://en.kioskea.net/faq/6088-excel...marks#solution
http://www.wfu.edu/~matthews/plus_mi...l_example.html
http://www.microsoft.com/education/h...s/grading.aspx

it would be better if you post question with example in detailed.

all the best