Answer
The number of fragments produced are 1.7 $\times 10^{25}$ .
Work Step by Step
From the earlier part of the question, we know that $8.58 \times 10^{24}$ fission events took place when the atom bomb underwent fission.
For every 235U nucleus that undergoes fission, 2 fragments are produced.
$\therefore$ Total number of fragments produced are
$$
2\left(8.58 \times 10^{24}\right)=1.72 \times 10^{25} \approx 1.7 \times 10^{25}
$$
