Amy has $$7$$ gray balls, $$4$$ white balls and $$3$$ black balls in a bag. What is the least number of balls she has to take out of the bag with her eyes closed to be sure that she takes at least two balls of each color? 
Overseas Competition->Knowledge Point->Combinatorics->Pigeonhole Principle->Worst Case in Pigeonhole Principle Problems
$$7+4+2=13$$． 