A motor ledger starts with a checked energy packet, not chemistry. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
A packet is only an energy source
One packet carries 12 joules in this checked training ledger.
12J
The packet does not describe chemistry
The diagram says energy available, not a reaction path; the checked value remains 12 joules.