<form action="#" method="post" data-ajax="false" onchange="output.value = (3.5 / 10) * paketscount.value " oninput="onchange()">
<select name="sitetime" id="sitetime" onchange="document.getElementById('paketscount').value=value; with (this) document.getElementById ('rez').value=value =
(selectedIndex) ? options [selectedIndex].text / 15 - 1 : '3'">
<option value="1" >60</option>
<option value="2" >90</option>
<option value="2" >120</option>
<option value="3" >150</option>
<option value="3" >180</option>
<option value="4" >210</option>
<option value="4" >240</option>
<option value="5" >270</option>
<option value="5" >300</option>
</select>
<div class="patets"><input style="width:50px!important; flaot:left;" type='text' id='paketscount' value="1" /></div>
<input id="rez" style="width:50px!important; flaot:left;" type='text' value="3" />
<output name="output">1</output>
</form>