If Z is the number of atoms in the unit cell that represents the closest packing sequence ABCABC....., the number of tetrahedral voids in the unit cell is equal to

The close packing sequence ABCABCABC... represent the cubic close packing structure. Here, Z is the number of atoms in the unit cell.
In ccp/fcc, there are two tetrahedral voids on the every body diagonal of the cube/unit cell.
Fcc has total 4 body diagonals. Thus, totally 8 tetrahedral voids present inside the unit cell.
Also, number of atoms in fcc/ccp arrangement is 4
Hence,
\(\text{No. of atoms per unit cell }= Z\)
\(\text{No. of tetrahedral voids }= 2Z\)