When 15cm of a gaseous hydrocarbon P, was burnt in 100cm of oxygen, gaseous mixture occupied 70cm at room temperature and pressure. When the mixture

When 15cm³ of a gaseous hydrocarbon P, was burnt in 100cm³ of oxygen, gaseous mixture occupied 70cm³ at room temperature and pressure. When the mixture was passed through potassium hydroxide, its volume decreased to 25cm³.
a). What volume of the oxygen was used during the reaction?
b).Determine the molecular formula of the hydrocarbon.