Gas stored in a tank at 273 k has a pressure of 50kpa. what happens to the volume when the pressure is decrease due to the added pressure

I can't think of any way that pressure can be decreased due to added pressure.

Be that as it may, and it certainly still is ...

No matter what you do to the pressure or the temperature of
the gas in that tank, the volume doesn't change.  Any sample
of gas always expands to fill the container in which it's confined. 
Its volume doesn't change until you either transfer it to another
container with different volume, or else change the volume of
the container it's in.