According to Boyle's law, the volume V of a gas at a constant temperature varies inversely with the pressure P. When the volume of a certain gas is 125 cubic meters, the pressure is 20 psi (pounds per square inch).

Question

If the volume of gas is increased to 400 cubic meters, the pressure will be

Answer 1

At constant temperature (which is what they want you to assume),
P*V = constant
125*20 = 400*P2
P2 = final pressure = (125/400)*20
= 6.25 psi

Since the equation
P1/P2 = V2/V1
involves dimensionless ratios, I did NOT have to convert pressure from psi to Pascal units and back again. I chose to keep pressure in psi and volume in cubic meters