A fixed optical train is clearer when several source signals pass through it. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Source twelve leaves output two through fixed transfers
Hold optical transfers at 1/2 and 1/3. Source 12 leaves detector signal 2.
12⋅21⋅31=2
Source eighteen leaves output three
Source 18 leaves output 3. The same two transfer stages still do the arithmetic.
18⋅21⋅31=3
Source twenty-four leaves output four
Source 24 leaves output 4. This is deterministic transfer bookkeeping, not photon counting noise.