A 55.0 kg bungee jumper steps off a bridge with a light bungee cord tied to her and to the bridge. The unstretched length of the cord is 15.0 m. She reaches the bottom of her motion 40.0 m below the bridge before bouncing back. Her motion can be separated into an 15.0 m free-fall and a 25.0 m section of simple harmonic oscillation. Use the principle of conservation of energy to find the spring constant of the bungee cord.