Select number in the web page and automatically (+, -, /, *) with the previous values. Also you can edit the function by your own.
This is a simple way to make math. Highlighting text in any website. The default function is sum, but you can easely change it jut press the other function (+, -, /, *). On the icon the color indicates which function is selected. For the most advanced users you can edit by yourself the equations by using the pop up page.