Boyle's law gives the relationship between pressure and volume of gas.
It states that at constant temperature, pressure is inversely proportional to volume of gas.
PV = k
where P - pressure, V - volume and k - constant
P1V1 = P2V2
where parameters for the first instance are on the left side and parameters for the second instance are on the right side of the equation
substituting these values in the equation
4.00 atm x 500 L = 8.0 atm x V
V = 250 L
new volume is 250 L