<script type="text/javascript">

function hesapla() 
{ 
var a=parseInt(document.not.txt1.value); 
var b=parseInt(document.not.txt2.value); 
var c=parseInt(document.not.txt3.value); 

var d=parseInt(a+b+c); 
var e=parseInt(d/3); 

switch (true) 
{ 
case (e>=0 && e<25): 
document.write("Not ortalamanız: 0"); 
break; 
case (e>=25 && e<45): 
document.write("Not ortalamanız: 1"); 
break; 
case (e>=45 && e<55): 
document.write("Not ortalamanız: 2"); 
break; 
case (e>=55 && e<75): 
document.write("Not ortalamanız: 3"); 
break; 
case (e>=75 && e<85): 
document.write("Not ortalamanız: 4"); 
break; 
case (e>=85 && e<100): 
document.write("Not ortalamanız: 5"); 
break; 
default: 
document.write("Hatalı değer girdiniz!") 
} 
}  
</script>

<form name="not"> 
<input type="text" name="txt1"> 
<input type="text" name="txt2"> 
<input type="text" name="txt3"> 
<input type="button" value="hesapla" onclick="hesapla()"> 
</form>