$3.$ In the gas$-$phase reaction, A$+$ B $->$C$+2$ D

it was found that when $2\mathrm{.}00$ mol A$,1\mathrm{.}00$ mol B$,$ and $3\mathrm{.}00$ mol D were mixed and allowed to come to equilibrium at $25\backslash$deg C$,$ the resulting mixture contained $0\mathrm{.}79$ mol C at a total pressure of $1\mathrm{.}00$ bar. Calculate $($a$)$ the mole fractions of each species at equilibrium, $($b$)$ Kx$,($c$)$ K$,$ and $($d$)\backslash$Delta rG$\backslash$deg $!!!!$ In this type of questions why and when we need to divide P total to P standard? where is this equation coming from and are there any unit of these equilibrium constants?