Atomic mass of Ne is 20.2. Ne is a mixture of Ne²⁰ and Ne²². Relative abundance of heavier isotope is:

Average isotopic mass
$$\begin{gathered} =\frac{\text{ per cent }\times \text{ mass of isotope }+\text{ per cent }\times \text{ mass of other isotope }}{100} \\ \begin{array}{l}\therefore 20.2=\frac{\mathrm{a}\times 20+(100-\mathrm{a})\times 22}{100}\\ \therefore \mathrm{a}=90;\\ \% \mathrm{of} \mathrm{lighter} \mathrm{isotope}=100-90=10\end{array} \end{gathered}$$