Question

In a closed packed array of N spheres the number of tetrahedral holes is

Answer 1

In a cubic closed packing arrangement of spheres, the number of tetrahedral holes is twice the number of atoms that make up the packing arrangement, and also twice the number of octahedral voids. Hence, if there are N spheres in the arrangement, there would be 2N tetrahedral voids.

Answer 2

The number of tetrahedral holes is twice the number of atoms in a closed packed structure. Therefore if there are N spheres in the arrangement, there would be 2N tetrahedral holes or voids. 
So answer is 2N.