Benny has $$4$$ blue marbles, $$3$$ yellow marbles and $$1$$ red one in his pocket.  How many marbles must be taken out from his pocket before he gets $$3$$ marbles of the same colour unquestionably? 
Overseas Competition->Knowledge Point->Counting Modules->Permutations and Combinations->Combinations Overseas Competition->Knowledge Point->Combinatorics->Pigeonhole Principle->Worst Case in Pigeonhole Principle Problems