help on excel project

hello
I need help on my computer class project
im calulating student test score which are number and converting them into letter grade using this formula...=IF(D7>=90,"A",IF(D7>=80,"B",IF(D7>=70,"C",IF(D7>=60,"D",IF(D7>=50,"F")))))

but on some student test score its missed meaning the student missed the test or inactive meaning the student drop the class.. i need a formulas to calculate the missed and inactive students score so for example for missed test score the answer should be "inc" for incomplete and for inactive it should be"drop"
i tried using this formula... =if(d7=missed,"inc) so if d7 test score is missed that means the total grade should be "inc" and for inactive student the test grade should be"drop"..i tried using that formula but an error message shows up like this #name?