Several friends use simple rope and pulley to raise a tree house from the ground into a tree. The moss of the tree heuse is 150kg. By pulling together, the friends manage exert an everage force of 1.6 10N as they raise the tree house a diatance of 3.2m above the ground. a) How much work did the friends actually do to raise the tree house? b) How much "useful" work was done? c) What is the efficiency of the rope and pulley in raising the treehouse? d) Suggest why the efficiency of this simple machine is not 100%