Dividing the sample output by the blank output cancels shared optical-chain losses, leaving a signal ratio rather than a concentration claim. With blank output 12 and sample output 4, the sample-to-blank ratio is 4/12 = 1/3 exactly for the stated toy normalization ledger; real blanks have drift, dark current, matrix effects, detector nonlinearity, and measurement uncertainty.
A sample-to-blank ratio is an instrument comparison. 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 and sample are instrument outputs
The blank output is 12 and the sample output is 4.
Sblank=12,Ssample=4
The ratio is a readout comparison
The sample-to-blank ratio is 1/3. It is a signal ratio, not a concentration claim.