Here are five number cards. 2,5,7,8,9. One of the cards is removed and the mean average of the remaining four number cards is 6

7

Step-by-step explanation:

Given that there are 5 number cards.

They are 2,5,7,8,9

Total of all numbers in the cards = 2+5+7+8+9 = 31

One card is removed say with number x.

Now sum new would be = 31-x ... i

Average of remaining 4 cards = 6

So sum of remaining 4 cards = 6 (4) = 24 ... ii

i and ii represent the sum of remaining 4 cards. Hence they should be equal

31-x = 24

Subtract 31 from both sides to get

-x=-7

Or x = 7