Answer:
13 nuclei
Step-by-step explanation:
From the question, the fission of one nucleus produces three neutrons which causes more nuclei to undergo fission.
This implies that, after the first fission, three neutrons cause three nuclei to undergo fission. The three nuclei that underwent fission produces nine neutrons which causes the fission of nine nuclei.
All together we the number of nuclei that underwent fission as;
1 + 3 + 9 = 13 nuclei.