Add "unknown" inputs for solving equations with missing variables

[AmityWilder]

When an input holds a value of ?, it could be used as a "for what value of this produces the result?"

For example: the parameters ${\color{red}A=5}, {\color{blue}B=?}, {\color{green}C=25}$ would result in the binary operations of:
$$\large \displaylines{ {\color{red}5} &+& {\color{darkorange} 20} &= {\color{green}25} \\ {\color{red}5} &-& {\color{darkorange}-20} &= {\color{green}25} \\ {\color{red}5} &\times& {\color{darkorange}5} &= {\color{green}25} \\ {\color{red}5} &\div& {\color{darkorange}\frac{1}{5}} &= {\color{green}25}}$$

and so on.