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.