Your code have numerous problems, the biggest one is that it is not an html page.
This code is too complicated
if(numbergrade >= 90){
document.getElementById('letter').value = "A+";
}
else if(89 >= numbergrade && numbergrade >= 85){
document.getElementById('letter').value = "A";
}
else if(84 >= numbergrade && numbergrade >= 80){
document.getElementById('letter').value = "A-";
}
...
which lead to simplifying
if(numbergrade >= 90){
document.getElementById('letter').value = "A+";
}
else if(numbergrade >= 85){
document.getElementById('letter').value = "A";
}
else if(numbergrade >= 80){
document.getElementById('letter').value = "A-";
}
...
This rewriting also solve another problem you have.
Your code gives grade 'F' if numbergrade=85.5
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