At equilibrium mole fraction of CO = 1/6, that of CH₃CH₃ = 1/6

so mole fraction of CH₃COCH₃  = 1- 1/6 - 1/6 = 2/3

let the total pressure at equilibrium = p

so pressure of CH₃CH₃ and CO will be p/6 and p/6

and that of CH₃COCH₃ = 2p/3 

now consider the following dissociation data

CH₃COCH₃ ----> CH₃CH₃ + CO

300 mm                        0                      0                            at starting

300-x  mm                    x                       x                            at  equilibrium 

here x = p/6  (1)  and 300-x = 2p/3   (2)                                         add these equations and get the value of p