MathDB

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

4-9

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 were there?

Problem Statement