As given C does half as much work as A and B together (A+B) can do it in 20 days (A+B) s 1 days wok =1∕20. A's 1 day work : B's 1 day Work =1∕2:1=1:2 A's 1 day work =(1∕20)×(1∕3)=(1∕60) [Divide 1∕20 in the ratio 1:2 ] B's 1 day work =(1∕20)×(2∕3)=1∕30 (A+B+C) 's 1 day work =(1∕60)+(1∕30)+(1∕40)=9∕120=3∕40 All the three together will finish it in 40∕3=13.33 days.