упс
<form class="search" action="" method="">
<select name="number" id="number" onchange="calc()">
<option>Number</option>
<option value="v1">1</option>
<option value="v2">2</option>
<option value="v3">3</option>
</select>
<select name="tools" class="tools" id="tools" onchange="calc()">
<option>Tool</option>
<option value="yes">yes</option>
<option value="no">no</option>
</select>
<div class="answer">
<span class="number">your number</span><span class="result" id="result">a</span>
</div>
</form>
span.number и span.result здесь display: block.
function calc() {
var number=document.getElementById('number').value,
tools=document.getElementById('tools').value,
result=document.getElementById('result');
if (number=="v1" & tools=="yes"){
result.innerHTML="b";
}
if (number=="v1" & tools=="no"){
result.innerHTML="$a";
}
if (number=="v2" & tools=="yes"){
result.innerHTML="d";
}
if (number=="v2" & tools=="no"){
result.innerHTML="c";
}
if (number=="v3" & tools=="yes"){
result.innerHTML="f";
}
if (number=="v3" & tools=="no"){
result.innerHTML="e";
}
}