class1 < data.frame(name=c("a1","a2","a3","a4","a5","a6","a7","a8","a9","a10"),
            surname= c("b1","b2", "b3", "b4", "b5","b6","b7","b8","b9","b10"),
            exam1= c(25,35,55,65,75,35,80,90,100,65),
            exam2 = c(56,65,25,95,78,69,45,23,45,63),
            final= c(65,55,78,93,65,89,89,95,100,75))
lastnote <- (((class1$exam1+class1$exam2)/2)*0.4 + (class1$final*0.6))
lastnote
class1[,"lastnote"] <- lastnote

for (i in lastnote)
if(lastnote[i] < 50) {x= "FF"
} else if(lastnote[i] < 60) {x= "CD"
} else if(lastnote[i] < 65) {x= "CC"
} else if(lastnote[i] < 70) {x= "BC"
} else if(lastnote[i] < 80) {x= "BB"
} else if(lastnote[i] < 90) {x= "AB"
} else {x= "AA"}