Using the ideal gas law: PV = nRT; and given the initial conditions:
V1 = 20 cc
T1 = 25 C or 298 K
We are asked to determine the volume at 0 C or 273 K. Assume that the pressure and number of moles are constant. Thus,
V/T = constant, then
V1/T1 = V2/T2
Solve for V2:
20 cc/ 298 K = V2/ 273 K
V2 = 18.32 cc