I actually got 1 physical chemistry-Equilibrium that I didn't know how to get the answer and the process/step needed to begin with. It's actually homework from my brother anyway. Poor thing, he was sick for a week and there's chemistry assignment due in the next day, he was already tried hard to solve other type of questions but in exceptional for this particular topic, even we both didn't know what strategy/hint required in order to solve this question. So, if possible, guys please leave some short explanation in your answer cause I may afraid that this kind of question would be appear in the exam. Thanks in advance.
1. Suppose that the equilibrium
4NH3(g) + 3O2(g) = 2N2(g) + 6H2O(g)
were established by adding 0.30 mol NH3 and 0.40 mol oxygen to a 5.0 L container. If "y" moles of water are present at equilibrium, then what would the concentration of oxygen be...

A. 0.40 - 0.50 y

B. 0.08 - 0.10 y

C. 0.30 - 0.50 y

D. 0.06 - 0.13 y

E. 0.40 - 3y/2

Answer is b.
1 mole of oxygen giver 2 moles of water.
So y/2 gives y moles of water.
Now remaining moles O2 is 0.4-y/2.
To get concentration divide by volume of container.