Let the efficiency of Bob be 3 units/day. So, Alex's efficiency will be 6 units/day, and Cole's will be 2 units/day. Since Bob can finish the job in 40 days, the total work will be 40×3=120 units. Since Alex and Bob work on the first day, the total work done =3+6=9 units. Similarly, for days 2 and 3 , it will be 5 and 8 units, respectively. Thus, in the first 3 days, the total work done =9+5+8=22 units. The work done in the first 15 days =22×5=110 units. Thus, the work will be finished on the 17 th day(since 9+5=14 units are greater than the remaining work). Since Alex works on two days of every 3 days, he will work for 10 days out of the first 15 days. Then he will also work on the 16th day. The total number of days =11.