You probably have heard about famous emacs `M-x calc`. I always wanted to become a poweruser of it but never could. My needs are too simple for it, I just want to quickly add/subtract numbers.
Recently I've discovered built-in `M-x quick-calc` which is perfect for what I need. It is already bound to `C-x * q`.
You can quickly calculate what's needed and the result is going to be saved to the kill-ring. You can paste the result in place if you want by prefixing the command with `C-u`.
Emacs has everything :)