The number of tetrahedral and octahedral voids in a $CCP$ unit cell are respectively:

$A$ compound is formed by two elements $M$ and $N$. Element $N$ forms a hexagonal closed pack $(HCP)$ array with $2/3$ of the octahedral holes occupied by $M$. The formula of the compound is $....$

In a close-packed arrangement,$A$ type of atoms are at corners,$B$ type of atoms are at face centers,$C$ type of atoms are at tetrahedral voids,and $D$ type of atoms are at octahedral voids. If all the atoms along any one body diagonal are removed,what will be the formula of the compound?

Calculate the packing efficiency of a metal crystal for a body-centred cubic $(BCC)$ structure.

What is the percentage efficiency of packing in $BCC$ structure (in $\%$)?

Fill in the blanks:
$(a)$ In $ccp$ structure,$a = \dots \dots r$.
$(b)$ In $hcp$ structure,packing efficiency $= \dots \dots \%$.
$(c)$ In $bcc$ structure,percentage of packing efficiency $= \dots \dots \%$.