The Q value is the leftover energy after the reaction ledger balances its starting and ending rest-energy totals. In this scaled training example, 30 joules before the reaction and 24 joules in the final rest ledger leave 6 joules available to release or carry away. Real nuclear Q values come from measured masses and uncertainties, so these joule numbers are a pinned arithmetic model rather than an isotope-table claim.
A Q value is the released part of an energy ledger. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Q value closes the reaction energy ledger
Initial energy 30 joules minus final rest 24 joules leaves 6 joules.
30−24=6J
No mass table is hidden in the number
The numbers are scaled training joules. The lesson does not use atomic mass units or real isotope tables.