A 360-mL sample of hydrogen is collected when the pressure is 800 mm Hg. What is the volume that the gas will occupy when the pressure is 720 mm Hg?

360/x=720/800
28800/720=720x/720
400-mL=x

Am I correct?

You're right but the - you have for 400-mL is not needed.

Yes, you are correct. The volume that the gas will occupy when the pressure is 720 mm Hg is 400 mL.

Yes, your calculation is correct. To solve this problem, you applied the principle of Boyle's law, which states that the volume of a gas is inversely proportional to the pressure it exerts.

You set up a proportion using the initial volume (360 mL) and pressure (800 mm Hg) and the unknown volume (x) and target pressure (720 mm Hg). By cross-multiplying and solving for x, you determined that the volume the gas will occupy when the pressure is 720 mm Hg is 400 mL.

So, your answer of 400 mL is correct.