There are 20 balls of green color,20 balls of red color,20 balls of blue color,20 of balck,20 of yellow. How many u have to pick to guarantee 2pairs of same color?
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It happens where you are sure what colour you're going to pick. How many balls of can you pick if you don't get any pair?

When you found this number, on the next pick, no matter what colour it is, you already have one ball of some colour already picked, so, you'll get your pair then. Can you post what you think is the answer? :)