 Okay, so we just figured out the percentage of carbon in carbon dioxide. Now let's figure out the percentage of oxygen. Well we can do this two ways. We can do the easy way because we know the percentage of carbon. We know that carbon dioxide is only composed of carbon and oxygen and we know the whole of anything has to be 100%. So the percentage of oxygen, the mass percent of oxygen is going to be 100, 0, 0, 0%, whatever, minus 27.292. You would expect that it would be the higher value both because oxygen is heavier and there is more of them. So that's going to give you the answer of 72.7 in carbon dioxide. Okay, so you could have done it that way but say you didn't have this calculation done or you hadn't done this one in a previous problem. So let's just erase this. People say we calculated all that other stuff though so I don't have to re-calculate it and we already wrote the formula for the mass of oxygen in carbon dioxide. So hopefully we get a number that's very similar to 72.7%. Anyways, so in order to do this you've got to remember that there's two oxygens in carbon dioxide. So you don't just want to put the mass of one oxygen here, this is the mass of all the oxygen in carbon dioxide. So in order to do that we're going to have to take 15.9994 multiplied by 2. Why? Because there's two of them. So we get 31.72.70835.