Answer
The number of neutrons released in environment are 1.3 x $10^{25}$
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, 2.5 neutrons are produced out of which 1 neutron will be used for the next reaction.
$\therefore$ We can consider that on avergae 1.5 neutrons are released in environment
$$
\left(8.58 \times 10^{24}\right)\times 1.5 =1.29 \times 10^{25} \approx 1.3 \times 10^{25}
$$
