Typically a scaffolding has two ropes to support it. One on the left most side while the other one is on the right most side. Based on equilibrium rule, the summation of forces must be 0. The downward force is the weight of person and scaffolding, while the upward force are the tensions on the two ropes, hence:
Weight person + Weight scaffolding = 2 * Tension
500 N + Weight scaffolding = 2 * 400 N
Weight scaffolding = 300 N