13. A can do a piece of work in 30 days while B can do it in 40 days. In how many days can A and B working together do it ?

14. A can do a certain job in 25 days which B alone can do in 20 days. A started the work and was joined by B after 10 days. The work lasted for:

15. 12 men or 18 women can reap a field in 14 days. The number of days that 8 men and 16 women will take to reap it, is: