Thermodynamics - Series Reaction - Estimating mols of reactants based on Free Energy of Rxn
What you do is write $[D]=[C]K_{CD}$, $[C]=[B]K_{BC}$, and $[B]=[A]K_{AB}$. So, $$[C]=K_{BC}K_{AB}[A]$$ and $$[D]=K_{CD}K_{BC}K_{AB}[A]$$So, $$[A]+[B]+[C]+[D]=(1+K_{AB}+K_{BC}K_{AB}+K_{CD}K_{BC}K_{AB})[A]=10$$