determine the volume of the residual gas at 20.0cm³ of hydrogen was sparked with 15.0cm³ of oxygen and the resulting mixture cooled at room temperature

Answer 1

2H2 + O2 ==> 2H2O
The coefficients tell you 2 mols H2 will use 1 mol O2 or
20 cc H2 will use 10 cc O2. So how much O2 remains?