Person A can do one-fifth of the work in 3 days, while B's efficiency is half of that of A. In how many days A and B working together can do half of the work?
A can do a work in \( 7 \frac{1}{2} \) hours and B can do the same work in \( 10 \frac{1}{2} \) hours. If they work together for \(3 \frac{1}{2}\) hours, then what part of the work will be left out?