C not sure 100 percent but my best guess
The decomposition of so3 to so2 gas and o2 gas can be described in the balanced chemical equation:
2so3(g) = 2 so2 (g) + 02(g).
so assuming a complete reaction, the ratio of so2 gas to total products is 2/3 while that of 02 is 1/3.
Subtracting water's water vapor pressure, 760-40 mm hG = 720 mm Hg.
then the products partial pressures are
so2 = 2/3 * (720) = 480 mm Hg.
o2 = 720-480 = 240 mm Hg.
