The energy budget accepts only when work and waste sum to input. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Input energy splits into two channels
Input energy is 60 joules, useful work is 36 joules, and waste heat is 24 joules.
36+24
Useful work plus waste closes input
The budget closes because useful work plus waste equals 60 joules.