problemas con formulario javascript

[mahonrry]

ya logre solucioanrlo talvez no de la mejor manera pero me funciono jajaja

Código:

<script type="text/javascript">
function div() {
            var txtFirstNumberValue = document.getElementById('txt1').value;
            var txtSecondNumberValue = document.getElementById('txt2').value;
            var result = Number(txtFirstNumberValue)     /     Number(txtSecondNumberValue);
            if (!isNaN(result)) 
            {
                document.getElementById('txt3').value = result;
            }
        }
function sum4() {
            var valor1 = document.getElementById('txt3').value;
            var valor2 = document.getElementById('txt4').value;
            var valor3 = document.getElementById('txt5').value;
            var result = Number(valor1) * Number(valor2) + Number(valor3);

            if (!isNaN(result)) 
            {
                document.getElementById('oc1').value = result;
            }
        }
function mul4() {
            var valor4 = document.getElementById('oc1').value;
            var valor5 = document.getElementById('txt5').value;
            var result = Number(valor4) * Number(valor5);

            if (!isNaN(result)) 
            {
                document.getElementById('txt7').value = result;
            }
        }


		</script>

<input type="text" id="txt1"   onkeyup="div();" size="8"/>
<input type="text" id="txt2"   onkeyup="div();" size="8"/>
<input type="text" id="txt3"   onkeyup="sum();" size="8"/>
<input type="text" id="txt4"   onkeyup="sum();" size="8"/>
<input type="text" id="txt5"   onkeyup="sum();" size="8"/>
<input type="text" id="txt6"   onkeyup="mul();" size="8"/>
<input type="text" id="txt7"   value="0"        size="8"/>
<input type="hidden" id="oc1"  onkeyup="mul();" size="8"/>