Exercise 9. In a deck of cards consisting only of red cards and black cards, there are 2 times as many black cards as red cards. If 4 black cards are added, there are then 3 times as many black cards as red cards. How many cards did the deck have before adding the 4 black cards?

Only a numerical answer is expected here.
Solution to Exercise 9 Let $r$ be the number of red cards and $n$ be the number of black cards in the initial deck. By hypothesis, we have $n=2 r$. After adding 4 black cards, there are $n+4$ black cards in the deck, and by hypothesis, we have $n+4=3 r$. Thus, $4=3 r-n=3 r-2 r=r$ and $n=2 \times 4=8$. Initially, the deck contains $r+n=4+8=12$ cards.

Graders' comments: The exercise was generally very well done. It's a pity that some students only found the number of black cards or made mistakes in their conclusion.