The audit distinguishes packet input from per-step and repeated work. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Packet total feeds the input budget
The packet source gives total input 60 joules, while the step-work source gives per-step work 12 joules.
60J input,12J per step
Repeated work feeds useful output
The repeated step source gives useful work 36 joules.