Let the pressure of the mixture = P and aqueous tension = p when the volume of cylinder is 4 L
When V = 2 L, total pressure = 2P + p = 15.
When V = 1 L, total pressure = 4P + p = 29
Solving, we get P = 7 atm, p = 1 atm.
When the volume = 8 L, total pressure = P/2 + p = 4.5 atm