On the table, 100 cards were lying with the white side up, each having one white side and one black side. Kostya flipped 50 cards, then Tanya flipped 60 cards, and after that, Olya - 70 cards. As a result, all 100 cards ended up lying with the black side up. How many cards were flipped three times?
Since all the cards ended up being flipped, each of them was flipped either once or three times. In total, there were 180 flips: 100 of them were needed to flip each card once; the remaining 80 were to flip some cards two more times. Therefore, 40 cards were flipped three times.

## Answer

40 cards.