You can represent vertical motion with dx/dt which will be constant, and vertical motion by dy/dt which is what you want to find, and you can relate x and y using the Pythagorean theorem. (You will use the info that waist is 1 metre off ground and that rope is 20 metres long; assume that tying the rope doesn't shorten it, i.e., following the rope from where the rope meets his waist to where the rope meets the object being lifted is 20 metres.)