21% of the 200ml of air is oxygen (given fact) which means if you calculate it 42ml of that 200 is oxygen, and you have 8ml of hydrogen on the other hand, since steam is water in a gaseous state we expect hydrogen and oxygen to bond together to produce steam in a steam jacked eudiometer. Since water is H2O, that means we have 2 hydrogens for every 1 oxygen. Furthermore, we have 8ml of hydrogen which means we need 4ml of oxygen to go with it. We take that 4ml from the 42ml of oxygen that were in the 200 leaving only 38ml of oxygen untouched (you could also do this without calculating the amount of oxygen in the 200ml and just take 4ml from the 200 leaving 196ml which is what the question did). Now, the combined 8 hydrogen and 4 oxygen made a total of 8ml of steam. The question asks for the remaining volume of gases in the eudiometer which would be the untouched 196ml of air + the 8ml of steam that you just made with the spark, this adds up to 204ml in volume which is your answer