Live fraction and source rate combine into an effective rate. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Long live time on a slow source gives one third hertz
Live time 8 seconds over clock 12 seconds gives live fraction 2/3.
Reff=1/2×2/3=1/3
Half-live clock gives one half hertz
Live time 6 seconds over clock 12 seconds gives live fraction 1/2.
Reff=1×1/2=1/2
Short live time on a fast source gives two thirds hertz
Live time 4 seconds over clock 12 seconds gives live fraction 1/3.