有用功:W有用=Gh=400Nx4m=1600J
总功:W总=FS=240Nx (4mx2)=1920J
额外功:W额外=W总-W有用=1920J-1600J= 320J
动滑轮的机械效率:η=W有用/W总=1600J/1920J=83.33%