Super smart! We have $$3 \, \mathrm{gm}$$ of Hydrogen and $$8 \, \mathrm{gm}$$ of Oxygen. To form water, $$3 \, \mathrm{gm}$$ of Hydrogen needs $$24 \, \mathrm{gm}$$ of Oxygen. But, we have only $$8 \, \mathrm{gm}$$ of Oxygen. So, the $$8 \, \mathrm{gm}$$ of Oxygen combines with $$1 \, \mathrm{gm}$$ of Hydrogen to produce $$9 \, \mathrm{gm}$$ of water. We will have $$2 \, \mathrm{gm}$$ of Hydrogen left over.