a group of monsters landed on earth and started reproducing by each splitting into two every hour. for any positive integer $k,$ we use $n k$ to denote the number of monsters $k$ hours after landing. we know that $n 0<5$ (so, fewer than $5$ monsters landed) and that the two digits at the right end of $n {100}$ are $28$. how many monsters landed on earth at the beginning?