$\mathrm{C}_2 \mathrm{H}_6+3.5 \mathrm{O}_2 \rightarrow 2 \mathrm{CO}_2+3 \mathrm{H}_2 \mathrm{O}$
$1 \mathrm{~mol}$ of ethane gives $3 \mathrm{~mol}$ of water
$0.1 \mathrm{~mol}$ ethane gives $0.3 \mathrm{~mol}$ of water
Number of molecules in $0.3 \mathrm{~mol}$ water
$=0.3 \times 6 \times 10^{23}=18 \times 10^{22}$ molecules
value of $x$ is 18