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"/>