Help with code

[Janfek]

I have this code to add some values and display a total:

<script language="JavaScript">
function updateTotal(intStart,intEnd,objTotalField) {
var numTotal = 0
var objForm = 'document.matprimas'
eval(objForm + "." + objTotalField +".value =0");
for (i=intStart;i<intEnd + 1;i++) {
numTotal -= eval(objForm + '.num'+ i +'.value');
}
eval(objForm + "." + objTotalField +".value =-numTotal");
var objForm =document.matprimas
objForm.total41.value = parseFloat(objForm.num25.value) + parseFloat(objForm.num35.value) + parseFloat(objForm.num45.value) + parseFloat(objForm.num55.value) + parseFloat(objForm.num65.value) + parseFloat(objForm.num75.value) + parseFloat(objForm.num85.value) + parseFloat(objForm.num95.value)
}
</script>

<td><input name="num1" type="text" id="num1" value="$d1_1" size="20" onChange="updateTotal(3,10,'total1');" /></td>
<td><input name="num2" type="text" id="num2" value="$d1_2" size="10" onChange="updateTotal(3,10,'total1');" /></td>
<td><input name="num3" type="text" id="num3" value="$d1_3" size="10" onChange="updateTotal(3,10,'total1');" /></td>
<td><input name="num4" type="text" id="num4" value="$d1_4" size="10" onChange="updateTotal(3,10,'total1');" /></td>
<td><input name="num5" type="text" id="num5" value="$d1_5" size="10" onChange="updateTotal(3,10,'total1');" /></td>
<td><input name="num6" type="text" id="num6" value="$d1_6" size="10" onChange="updateTotal(3,10,'total1');" /></td>
<td><input name="num7" type="text" id="num7" value="$d1_7" size="10" onChange="updateTotal(3,10,'total1');" /></td>
<td><input name="num8" type="text" id="num8" value="$d1_8" size="10" onChange="updateTotal(3,10,'total1');" /></td>
<td><input name="num9" type="text" id="num9" value="$d1_9" size="10" onChange="updateTotal(3,10,'total1');" /></td>
<td><input name="num10" type="text" id="num10" value="$d1_10" size="10" onChange="updateTotal(3,10,'total1');" /></td>
<td>&nbsp;</td>
<td><input name="total1" type="text" id="total1" value="$d1_11" size="10" /></td>

waht this does is adds 7 values of input and display a total in a different input.

What I need to add is a select and another input, and when I select one of the options and when a value is entered in that input I want to see those 2 values multiplied in the input I have as num3.

How do i do this?
Thanks in advance.