Number of atoms in $6\mathrm{g} \mathrm{of} {\mathrm{H}}_2\mathrm{O}$ is:

$\mathrm{Moles}=\frac{6}{18}\times 6.023\times {10}^{23}=\frac{1}{3}\times 6.023\times {10}^{23}\times 3=6.023\times {10}^{23}$

Number of atoms present in $0.4 \mathrm{g} \mathrm{He}$

$\mathrm{Moles}=\frac{0.4}{4}\times 6.023\times {10}^{23}=\frac{1}{10}\times 6.023\times {10}^{23}=\frac{6.023\times {10}^{23}}{10}$

Number of atoms present in $22 \mathrm{g} {\mathrm{CO}}_2$

$\mathrm{Moles}=\frac{22}{44}\times 6.023\times {10}^{23}=\frac{1}{2}\times 6.023\times {10}^{23}=\frac{6.023\times {10}^{23}}{2}$

Number of atoms present in $1 \mathrm{g} {\mathrm{H}}_2$

$\mathrm{Moles}=\frac{1}{2}\times 6.023\times {10}^{23}=\frac{1}{2}\times 6.023\times {10}^{23}\times 2=6.023\times {10}^{23}$

Number of atoms present in $12 \mathrm{g} \mathrm{CO}$

$\mathrm{Moles}=\frac{12}{28}\times 6.023\times {10}^{23}=\frac{3}{7}\times 6.023\times {10}^{23}$