Answer
4.88 rev/s
Work Step by Step
In this case, there is no external torque. So its angular momentum is conserved.
We can write,
$I_{1}\omega_{1}=I_{2}\omega_{2}$
$\omega_{2}=\omega_{1}(\frac{I_{1}}{I_{2}})$; Let's plug known values into this equation.
$\omega_{2}=1.66\space rev/s\times(\frac{3.56\space kgm^{2}}{1.21\space kgm^{2}})=4.88\space rev/s$
