Several measured and blank pairs show that net signal is computed, not assumed. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Blank subtraction turns eighteen into twelve
Measured signal 18 includes blank 6, so the net sample signal is 12.
18=6+12
A different blank changes the same measured scale
Measured signal 20 with blank 5 leaves net 15. This row uses the same checked subtraction rule.
20=5+15
Higher measured and blank still close exactly
Measured signal 24 and blank 8 close at net 16. Three rows make the background rule visible.