Código:
function sum() {
var txtFirstNumberValue = document.getElementById('box1').value;
var txtSecondNumberValue = document.getElementById('box2').value;
var result = parseInt(txtFirstNumberValue) / parseInt(txtSecondNumberValue);
if (!isNaN(result))
{
document.getElementById('box3').value = result;
}
}
<input type="text" id="box1" value="0" onkeyup="sum();" /><br/>
<input type="text" id="box2" value="0" onkeyup="sum();" /><br/>
<input type="text" id="box3" value="0"/>



