How shall I have finished this work?

A started a work and left after working for 4 days, then B was called and he finished the work in 18 days. If A left the work after working for 6 days then B would have finished the remaining work in 12 days. In how many days can each of them, working alone finish the whole work?

Mr. Rajesh is twice as good a worker as Mr. Vishal and together they finish a piece of work in 28 duys. In how many days will Vishal alone finish the work?

If 40 men can finish a piece of work in 60 days. In how many days will 75 , men finish the same work ?

A is twice as good a workman as B and together they finish a piece of work in 18 days. In how many days will A alone finish the work?

A is twice as good a workman as B and together they finish a piece of work in 20 days. In how many days will A alone finish the work?