SS2 Gradebook

This assignment took a lot of hard work and determination. For some reason I'm having a lot of problems figuring out the functions and applying them to the grade book. However, I think I finally figured out everything, except I’m not sure about the letter grades. I used the substitute formula rather than the IF formula, because I couldn't get it to work. I hope that this is what you’re looking for. I thought I knew how to use Excel, but these past two assignments have really made me doubt that opinion. Here is a link to Spreadsheet 2.