In the unit cell of NaCl (fcc Cl-, Na+ at the edge centres and body centre), all Cl- ions at the face centres are removed and nothing else is changed. The simplest formula of the resulting solid is

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In the unit cell of $NaCl$ (fcc $Cl X^{-}$ , $Na X^{+}$ at the edge centres and body centre), all $Cl X^{-}$ ions at the face centres are removed and nothing else is changed. The simplest formula of the resulting solid is

Remaining

Cl X^{-}

are only the 8 corners:

8 \times \frac{1}{8} = 1

Na X^{+}

: 12 edge centres

\times \frac{1}{4} = 3

plus 1 body centre

= 4

. The ratio is 4 : 1, so the formula is

Na X_{4} Cl