A horizontal rope is tied from a house to a pulley and down to a paint bucket filled with sand. The paint bucket normally keeps the rope taught and it doesn't sag. A 450g bird lands on the rope (effectively turning it into two different roped). Measured horizontally, the bird is 4m from the house, and 2m from the pulley. If the bird deflects the rope downward by 18cm at its location, what is the mass of the bucket of sand?