<script>
var b = 28.35;

function func(el,action,res)
{
a = document.getElementById(el).value;
c = 0;
switch (action)
{
case "*": c = a * b; break;
case "/": c = a / b; break;
default: c = 0;
}
document.getElementById(res).innerHTML = c;
}
</script>

<input type="text" id="a1" value="0" name="a1"/><input type="button" onclick="func('a1','*','res1');" value="="/><span id="res1">0</span>
<br/>
<input type="text" id="a2" value="0" name="a2"/><input type="button" onclick="func('a2','/','res2');" value="="/><span id="res2">0</span>