This problem can be solve by using this formula:
V1 x P1 = V2 x P2
initial volume x initial pressure = final volume x final pressure
Our initial volume is 75 mL, initial pressure is 300 atm, final pressure is 200 atm.
note: I assume that the unit of pressure in this problem is atm since mL is the unit of volume.
We are looking for V2 which is the final volume.
Let's start solving this;
V1 x P1 = V2 x P2
75 x 300 = V2 x 200
22500 = V2 x 200
22500 / 200 = V2 x 200 / 200
112.5 = V2
V2 = 112.5 mL
Therefore, the final volume of a gas is 112.5 mL.
