From 320 mg. of ${\mathrm{O}}_2,$ $6.023\times {10}^{23}$molecules are removed, the no. of moles remained are

Mass of oxygen given = 320 mg

Mass of $6.023\times {10}^{23} \mathrm{molecules} =32 \mathrm{gram}$

Mass of $6.023\times {10}^{23} \mathrm{molecules} =\frac{32}{6.023\times {10}^{23}}\times 6.023\times {10}^{20}$

$=32\times {10}^{-3}\mathrm{g}$

$=32\mathrm{mg}$

Mass of gas remains = 320 − 32

= 288 mg

No .of moles gas = $\frac{288}{32}\times {10}^{-3} moles$

$=9\times {10}^{-3} \mathrm{moles}$