Let the total amount of work be 60 units. Then Anu, Vinu, and Manu do 4, 5, and 3 units of work per day respectively. On the 1st day, Anu and Vinu work. Work done on the 1stday=9units On the 2nd day, Manu and Vinu work. Work done on the 2ndday=8units This cycle goes on. And in 6 days, the work completed is 9+8+9+8+9+8=51units. On the 7th day, again Anu and Vinu work and complete the remaining 9 units of work. Thus, the number of days taken is 7 days.