BALTIC-WAY 1997 Problem 20
- Answer
Twelve cards lie in a row. The cards are of three kinds: with both sides white, both sides black, or with a white and a black side. Initially, nine of the twelve cards have a black side up. The cards 1-6 are turned, and subsequently four of the twelve cards have a black side up. Now cards are turned, and six cards have a black side up. Finally, the cards 1-3 and 10-12 are turned, after which five cards have a black side up. How many cards of each kind are there?
Dataset text NuminaMath olympiads. Quoted with source.
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9 \text{ cards with one black and one white side and 3 cards with both sides white}
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