(1) Useful work is equal to the work done by directly lifting an object 2m at a constant speed;

W useful = GH =100n× 2m = 200J;

(2) The total work done by pulling force W is total = FS = 50n×2×5m = 500J；;

(3) Pull power P=W, total t=500J5s= 100W.

(4) The mechanical efficiency of this inclined plane η=W, which is always useful w×100% = 200J500J×100% = 40%.

So the answer is:100; 40%.