If there was a key that could unlock anything and a lock that could lock anything without being unlocked, what would happen if I used the key to unlock the lock?
You have 19 fishes, you had 10, then 4 died -> 6, then 3 came back to life -> 9 then, the ones alive double their number -> 18, then another 1 because the one who died and didn't come back -> 19
