There are 5 white, 7 red and 8 black balls in a box. If you are blindly picking up balls from the box, what is the minimum number of balls you need to take out of the box to ensure that you have at least 2 balls of the same color?

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Correct Answer: C. 4.
After the first ball is picked, the second may not be the same color. After two balls picked, the third may still be a different color than the first two (since there are a total of 3 colors). But when the fourth ball is picked, it must match one of the prior colors. Therefore, picking 4 balls will ensure that you have at least one set of 2 balls with the same color.
The numbers 5, 7, 8 do not matter here (they could matter if these numbers were lower, or the number of balls picked were higher).
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