Answer: 7 (the 3rd card) was removed.
Explanation:
How to get the mean average: Add all the numbers together to get the sum then divide the sum by however many numbers there are.
If you find the mean average for 4 cards by removing a number/card each time, once you get to removing 7, you will have 2 + 5 + 8 + 9.
2 + 5 + 8 + 9 = 24
24/4 = 6.
I hope this helps!
Also, if I get this wrong, please leave a comment and tell me why.