You can make use of storeEval command in Selenium IDE. Using it you can add, divide, multiply any variable with desired value and then use it.
Command: storeEval Target: storedVars['delay']*2 Value: doubleDelay
<body>
<table cellpadding="1" cellspacing="1" border="1">
<thead>
<tr><td rowspan="1" colspan="3">New Test</td></tr>
</thead><tbody>
<tr>
<td>store</td>
<td>5000</td>
<td>delay</td>
</tr>
<tr>
<td>storeEval</td>
<td>storedVars['delay']*2</td>
<td>doubleDelay</td>
</tr>
<tr>
<td>echo</td>
<td>${doubleDelay}</td>
<td></td>
</tr>
</tbody></table>
</body>
http://www.questionselenium.com/2012/10/echo-storeeval-and-storedvars.htmlreference for storeEval