Quick Mode is a quick way to use Calc when you don't need the
full complexity of the stack and trail. To use it, type M-# q
(quick-calc) in any regular editing buffer.
Quick Mode is very simple: It prompts you to type any formula in standard algebraic notation (like `4 - 2/3') and then displays the result at the bottom of the Emacs screen (3.33333333333 in this case). You are then back in the same editing buffer you were in before, ready to continue editing or to type M-# q again to do another quick calculation. The result of the calculation will also be in the Emacs "kill ring" so that a C-y command at this point will yank the result into your editing buffer.
Calc mode settings affect Quick Mode, too, though you will have to go into regular Calc (with M-# c) to change the mode settings.
See section "Quick Calculator" Mode, for further information.