the answer is .00620 moles but i don't know why.?

Question

A reaction is carried out under an atmosphere of oxygen. As the reaction proceeds, oxygen is
consumed. The reaction started in a 500.0 mL flask under a pressure of 1.34 atm and a
temperature of 22.0 °C. How many moles of oxygen had been consumed if at the end of the
reaction the pressure was 1.04 atm and the temperature did not change?

Answer 1

Applying the idea gas law: PV = nRT
Thus n = (1.34 - 1.04)*0.5/(0.08206*295) = 0.00620 (mol)