GRE study help-classical physics

[lifes3ps]

last problem with classical- thermo last night was a breeze.

problem:
child on edge of solid disk (merry goround)
M(child)=40kg
M(disk)=200kg
r(disk)=2.5m
w(disk)=2.0rad/s

child moves to center, whats final angular velocity of merry go round?, neglecting size of child.

my work:
here i add the mass of the child and disk for I=kmr^2.
K=0.5 for disk, thus I=(1/2)*(240)*(2.5)^2=750.

L=Iw=750*2=1500.

conserving L, changing I
I'=1/2*200*2.5^2=625
L'=I'w', 1500=625*w' -> w'=2.4 [rad/s]. which is not the answer

correct: 2.8 [rad/s], where am i going wrong?

**note, if i didnt say above, child starts on edge of disk, @r=2.5m