If ${10}^{21 }$molecules are removed from 200mg of $C{O}_2$ the number of moles of  $C{O}_2$ left are

$200mgC{O}_2=0.2gr=\frac{0.2}{44}mol$

$0.00454mole\Rightarrow 4.54\times {10}^{-3}mol$

${10}^{21}\mathrm{molecules} \mathrm{of} {\mathrm{CO}}_2=\frac{{10}^{21}}{6.02\times {10}^{-23}}=1.66\times {10}^{-3}\mathrm{mol}$

No of moles Left

$=(4.54-1.66){10}^{-3}=2.88\times {10}^{-3}$