to do this with one table you need to introduce more columns and then get the column widths by using colspan
so you can get the combined widths you require
to get this to work well across browser you will possibly need to use the <col> and <colgroup>
elements : http://www.w3.org/TR/html401/struct/tables.html#h-11.2.4
<body bgcolor="#14B3D9">
<table width="100%" border="1" bgcolor="#ffffff">
<colgroup>
<col width="25%">
<col width="25%">
<col width="25%">
<col width="5%">
<col width="20%">
</colgroup>
<tr>
<td>25</td>
<td colspan="2">50</td>
<td colspan="2">25</td>
</tr>
<tr>
<td colspan="2">50</td>
<td colspan="2">30</td>
<td>20</td>
</tr>
</table>
</body>