The average of a set of whole numbers is 27.2. when the 20% of the elements are eliminated from the set of numbers then the average become 34. The number of elements in the new set of numbers can be :

only (c) is correct since it is divisible by 4.

Let the original number of element  be x then the new no.of elements will be 

            $\frac{4x}{5} = K$                               

So K must be divisible by 4

Since,      x =  Kx5/4