<script  Language="JavaScript"> 
function hideCalc() {
	document.getElementById("calcHolder").style.display = "none";
	document.getElementById("questHolder").style.display = "none";
}
window.onload = hideCalc;
function calculate() 
{ 
 answer.innerHTML=calc.a.value*calc.b.value; 
} 
function question() {
	document.getElementById("questHolder").style.display = "";
}
function showCalc() {
	document.getElementById("calcHolder").style.display = "";
}
</script> 
<select name="works" onchange="question()">  
    <option value="13">кровля</option>  
    <option value="8">наружный фасад</option>  
    <option value="99">внутренняя отделка</option>  
</select>  
<div id="questHolder">
	<select name="type" onchange="showCalc()">  
    <option value="13">гидроизоляция</option>  
    <option value="8">пароизоляция</option>  
</select>  
</div>
<div id="calcHolder">
<form name="calc">  
<select name="a">  
    <option value="13">рубероид</option>  
    <option value="8">пергамин</option>  
    <option value="99">изоэласт</option>  
    <option value="119">техноэласт</option>  
    <option value="35">бикрост</option>  
</select>  
<input type="text" value="1" name="b" size="12">м<sup>2</sup>&nbsp 
<input type="button" value="Расчитать" onclick="calculate()"></form><hr><span id="answer">0</span> руб  
</div>