How much work done if only half of the rope is pulled to the top of the building?

Question

4. A soft long rope with a weight of 3lb/ft hangs over the edge of a building 200ft high. How much work done if only half of the rope is pulled to the top of the building?

Answer 1

Work is equal change of potential energy of rope: W = (100·3·150 + 100·3·200) - 200·3·100 = 45000 lb·m