1. A calculation device comprising: entry keys configured to allow a user to enter a mathematical expression including at least one variable; a display configured to display the mathematical expression; a hardware processor, wherein the processor: recognizes when the mathematical expression is being entered, recognizes the at least one variable of the mathematical expression, and stores, in a memory, at least a first variable of the at least one variable; and a shortcut key for entering one of a list of previously entered variables.