This is a boyles law question. You have to use the equation P1V1=P2V2, where P is pressure and V is volume, and 1 represents initial values while 2 represent final values. So, in this instant P1=80kPa, V1=500L, and P2=400kPa. Since you know all except one variable, you can plug them into the boyles law equation to solve for V2.
(80)(500)=(400)V2
V2=100L
So the new volume is 100 liters. This makes sense because you would expect the volume to decrease quite a bit since the pressure has increase so much.
