I was struggling to solve a thermodynamics problem, until I looked up the page with solutions and found something that makes absolutely no sense to me. Basically, I'm given a balloon with a certain volume, pressure and amount of hydrogen. The amount of hydrogen is then reduced, as well as the pressure. Temperature is constant. Now, what would make sense to me is that, if I had a balloon with a certain volume, pressure and amount of substance, and if I reduced the substance and the pressure - volume would also be reduced, if the temperature is constant. Since the gas is what keeps the balloon bloated, if you have less of it and it makes smaller pressure to the balloon, I guess that the volume would also lessen. But no, the solution says that the volume is also constant, together with the temperature. Can somebody explain to me how is this possible? Or is it a mistake in the textbook?