Well, let's see. Let's first notice that the ratio of the number of moles of methane, the number of moles of oxygen is one to two. So that means whenever 1 mol reacts with two moles of this guy, they produce carbon dioxide, 1 mol and two moles of water. So now we have two moles of methane reacting with oxygen. That means, since not one, but two moles of methane are reacting, that means not two, but four moles of oxygen will react to produce our products. That's because two times two is four.