If a person can do a piece of work in "m" days, he can do 1/m of the work in 1 day.
If the number of persons engaged to do a piece of work be increased (or decreased) in a certain ratio the time required to do the same work will be decreased (or increased) in the same ratio.
If A is twice as good a workman as B, then A will take half the time taken by B to do a certain piece of work.
Time and work are always in direct proportion.
If two taps or pipes P and Q take "m" and "n" hours respectively to fill a cistern or tank, then the two pipes together fill (1/m + 1/n) part of the tank in 1 hour and the entire tank is filled in (m*n)/(m+n) hours.