A sample of helium gas has a volume of 2.0 L at a pressure of 4.0 atm. What is the volume of gas at 380 mmHg

From equation;

P1V1=P2V2

V2=P1V1:P2

since P2=380mmHg

now; 1atm=760mmHg

how about 380mmHg is equal to how many atm?

380*1:760=0.5atm

P2 now is equal to 0.5atm

back from equation;

P1V1=P2V2

V2=P1V1:P2

V2=4.0atm*2.0L:0.5atm

V2=16L

therefore V2=16L.