Seven girls can do a piece of work in thirteen days. six boys can do the same piece of work in twelve days. nine men can do the same piece of work in nine days and six women can do the same piece of work in fourteen days. Who are the most efficient?

Seven girls can do a piece of work in thirteen days.
One girl will do the work in 13*7 = 91 days.
Six boys can do the same piece of work in twelve days.
One boy will do the work in 12*6 = 72 days.
Nine men can do the same piece of work in nine days.
One man will do the work in 9*9 = 81 days.
 Six women can do the same piece of work in fourteen days.
One woman will do the same work in 14*6 = 84 days.

The most efficient is the one who takes the least amount of time to complete the work i.e. Boys.