a) Notice that $kn + k + n = (k + 1)(n + 1) - 1$, $3 + 1 = 2^2, 24 + 1 = 5^2$, so once we perform the step, it leads to a square number - 1. Essentially, all numbers will be a square number - 1, since $k + 1, n + 1$ are perfect squares. However, $10^7$ isn't a perfect square so it isn't possible. b) Perform the step on $3, 24$ to give $99$, repeat with $99$ to obtain $99999999$. c) Since $4, 25$ have only $2$ and $5$ as their prime divisors, every number will be of the form $2^a5^b - 1$, so we can't obtain $48999999$.