Using data ranges in an IF statement

hey anyone!

can anyone help me with my problem?

I need to be able to give a grade A, B, C or a D by using a range of scores in an IF function for a markbook as part of my IT coursework.

I have had several failed attempts, and this is one of my many formulas that has failed:

=IF(O6<45,"D",IF(O6=$46:$60,"C",IF(O6<$61:$74,"B",IF(O6>75,"A"))))

can anyone tell me what is wrong with it?

PS. I have attached a screenprint.

thankyou.