A playground carousel is free to rotate about its center on frictionless bearings, and air resistance is negligible. The carousel itself (without riders) has a moment of inertia of 140 kgm2. When one person is standing at a distance of 1.40 m from the center, the carousel has an angular velocity of 0.410 rad/s. However, as this person moves inward to a point located 0.850 m from the center, the angular velocity increases to 0.550 rad/s. What is the person's mass?

I know IW=IW but i don't know where to go from there?