What is the simplest formula of a solid whose unit cell has the...

An atom at the corner of a cube is shared among 8 unit cells. As there are 8 corners in a cube, number of corner
atom $\left(A\right)$ per unit cell $8 \times \dfrac{1}{8} = 1$
Face-centred atom in a cube is shared by two unit cells.
As there are 6 faces in a cube, number of face-centred
atom $\left(B\right)$ per unit cell $6 \times \dfrac{1}{2} = 3$
An atom in the body of the cube is not shared by other cells.
$∴$ Number of atoms $\left(C\right)$ at the body centre per unit cell = 1.
Hence, the formula of the solid is $A{B}_{3}C$ .