5 is the number of moles of gas produced (4 of NO2 +1 of O2), dont count the other product because it's a solid,and the 2 is the number of moles used. So for every 2 moles of Sr (NO3)2 there are 5 moles of gas produced. Hence calculating the moles of gas produced and putting the values into the equation for volume gives the volume of gas obtained by student.