Time and Work - General Questions

A, B and C can do a piece of work in 18, 6 and 9 days respectively. Working together, how long will they take to finish it?

A can finish a work in 26 days and B in 13 days. They work together for 6 days and then A leaves. In how many more days will B finish the remaining work?

A can do a piece of work in 7 days and B can do it in 22 days. In how many days will they finish it working together?

A can do a piece of work in 21 days and B can do it in 14 days. In how many days will they finish it working together?

A can finish a work in 11 days and B in 11 days. They work together for 4 days and then A leaves. In how many more days will B finish the remaining work?