Machine P       60 mins         40 posters

                            1 min             2/3 poster

Machine Q      60 mins         90 posters

                             1 min              3/2 poster

u = time they were both printing

 20 + (2/3) u = (3/2)u

(9-4) u/ 6   = 20

u = 120/5 = 24 mins

Machine P      24 mins       (2/3) x 24 = 16        plus 20 printed earlier  =>  36

Machine Q      24 mins       (3/2) x 24 = 36

Hence total copies = 36 + 36 = 72

PS: Aaron, please click on “Accept Answer” if the answer is correct.