Answer:
7
Step-by-step explanation:
If one of the cards is removes and the mean average of the remaining four number cards is 6, then their sum is [tex]6\cdot 4=24.[/tex]
The sum of all five cards is
[tex]2+5+7+8+9=31.[/tex]
Since [tex]31-24=7,[/tex] then the card with number 7 is removed.