Child on a Merry-Go-Round: Calculating Angular Velocity After Moving Inward

[Steveadams79]

Homework Statement

A child with a mass of 40kg is sitting 2m from the center of a merry go round. He moves inward to a point 1.5m from the center. If the merry go round was originally spinning at 30rpm's, how fast is it spinning after the child moves inward?

Homework Equations

The Attempt at a Solution

 

[LowlyPion]

Welcome to PF.

How would you think to approach the problem?

 

[Steveadams79]

I really do not have a clue. i can't seem to find which formula I need to use.

 

[RoyalCat]

What is the initial state?
What is the final state?
The preservation of what quantities holds here?

 

[jgens]

Hint: No net external torque acts on the system.