If you need more from your mobile's pathetic free calculator than adding and subtracting it's probably time you got yourself some serious scientific capability. And Free42 really is a very serious scientific calculator, mainly due to the fact that it's based on the Hewlett Packard 42S.
Think of an equation, any equation. Because the Free42 can handle pretty much anything - linear, statistics, matrices, functions with graphical representations. The calculator displays itself on your mobile exactly as it exists for real, however it can all appear a bit small as if squeezed into the screen. In terms of what it offers though this is a very good solution to all your mathematical concerns.