A 75 kg skier is going downhill so fast that he cannot brake on time and ends up smashing against an inflatable guardrail and bouncing back in the air. Fortunately, he was wearing his helmet and survived without serious injuries. Considering that the skier started with no impulse, the ski track friction and air resistance are negligible, g = 10m/s², h₁ = 20 m, and h₂ = 2.00 m, please calculate the guard rail coefficient of restitution (in %).