To solve this we assume that the gas is an ideal gas. Then, we can use the ideal gas equation which is expressed as PV = nRT. At a constant temperature and number of moles of the gas the product of PV is equal to some constant. At another set of condition of temperature, the constant is still the same. Calculations are as follows:
P1V1 =P2V2
P2 = P1V1/V2
P2 = 1 atm x 450 mL / 2000 mL
P2 = 0.225 atm