There is a sample IF function in the HELP menu that will do exactly what you
want. For example, if your final grade is in cell F1 you can put a formula
in G1 that says:
All you have to do is to change the "F1" to what cell your grade is in. The
easiest way to see the sample is to click in your cell, click the "Fx"
(Insert Function) icon on the Formula Bar, click the "IF" statement to
highlight it, then click "Help on this function" at the bottom of the dialog
box. This example is in that HELP menu topic.