Final answer:
The number of neutrons would reach half a billion after 9 minutes, since the number of neutrons doubles every minute and it reached one billion after 10 minutes.
Step-by-step explanation:
The question involves an understanding of exponential growth and the concept of half-life, which is more commonly associated with radioactive decay. The number of neutrons in the reactor doubles each minute. If after 10 minutes there are one billion neutrons, then after 9 minutes there would be half that amount, because the doubling time is one minute. Therefore, the number of neutrons reached half a billion after 9 minutes.
The number of neutrons reached half a billion in 9.75 minutes.