If A can do a piece of work in n days, then A's 1 day work = 1/n.
If A's 1 day work = 1/n, then A can finish the work in n days.
If A is twice as good a workmen as B, then
Ratio of work done by A and B in the same time = 2:1
Ratio of time taken by A and B in doing the same work = 1:2