A can complete a work in 12 days and B can do the same work in 21 days. In how many days they complete the work working together?
1
84/9 days
2
84/11 days
3
84/13 days
4
84/15 days
A can complete a work in 12 days and B can do the same work in 21 days. In how many days they complete the work working together?