There are two stacks of stones, A and B, each containing $$20$$ pieces of stone. Tom and Jerry take stones out of the stacks one after another. In each turn, each player can take an unlimited number of stones from one of the stacks. To end their turn, each player~\textbf{MUST} pick at least one piece of stone each round, and the player who picks the last piece of stone wins the game.  If Tom takes $4$ stones from Stack B first, find out from which stack and how many stones Jerry must take to win the game.  Answer: Jerry must take~\uline{~~~~~~~~~~}~stone(s) from Stack~\uline{~~~~~~~~~~}~. 
Overseas Competition->Knowledge Point->Number Theory Modules