This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
As far as I'm aware, there are no taglibs for mathematical operations in the Struts framework. You'll have to perform the operation between scriptlet tags, or (the better solution) is perform anything you need within the action that's the predecessor for your jsp page. You jsp should, in theory, not need to perform such calculations, merely present the results.