Time And Work

A is twice as good a workman as B. Together they finish a piece of work in 24 days. In how many days B alone can finish the work?

Ten men working 8 hours a day can do a work in 15 days. How many more men need to be employed to do the same work in 10 days. If everyone works 10 hours per day?

A and B together can do a work in 30 days. they worked together for 20 days and then B left A alone finished the remaining work in 20 days. In how many days A alone can do the whole work?

A and B can do work in 12 days and 24 days respectively. They started working together but left the work 3 days before its completion and thereafter B alone completed the work. For how many days A worked with B?