11. A deck of playing cards has four suits, each with 13 cards, from which cards are drawn at random. Question: What is the minimum number of cards that must be drawn to ensure that 4 cards are of the same suit?
13
11. 【Solution】Selecting 3 cards of each suit, a total of 12 cards, it is clear that drawing 12 cards cannot guarantee that there are 4 cards of the same suit.

If 13 cards are drawn, since there are only 4 suits, one of them must have more than 3 cards, i.e., there must be 4 cards of the same suit.

Answer: At least 13 cards must be drawn to ensure that there are four cards of the same suit.