<form name="MyForm" action="send.php" method="post">
<table>
<tr>
<td><select name="num" onchange="with (this) document.getElementById ('rez').innerHTML =
(selectedIndex) ? options [selectedIndex].text * 100 + ' руб.' : ' '">
<option>выберите количество</option>
<option >1</option>
<option >2</option>
<option >3</option>
<option >4</option>
<option >5</option>
</select></td>
<td id="rez"> </td>
</tr>
</table>
</form>
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