Thank you for the opportunity to help you with your question!

So if you decrease the temperature, then something else has to change in the system. You are told that the pressure remains constant.

That means that the only other option for change would be the volume.

Volume and temperature are directly proportional (PV = nRT). They are on opposite sides of the equation so that when one increases, the other will have to increase as well.

So if the temperature is reduced by 1/2, then the volume will be reduced by 1/2 as well.

Please let me know if you need any clarification. I'm always happy to answer your questions.