The angular-momentum ledger is a checked product. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Angular momentum is inertia times spin rate
Inertia 6 times spin rate 4 gives angular momentum 24.
L=6×4=24kgm2/s
Spin rate is shown as per second
The spin label is 4 per second, not hertz or a hidden radian unit.