40% of the people read newspaper X. 50% read newspaper Y and 10% read both the papers. What percentage of the people read neither newspaper?

$\begin{array}{l}\mathrm{n}\left(\mathrm{A}\right)=40,\mathrm{n}\left(\mathrm{B}\right)=50,\mathrm{n}(\mathrm{A}\cap \mathrm{B})=10\\ \mathrm{n}(\mathrm{A}\cup \mathrm{B})=\mathrm{n}\left(\mathrm{A}\right)+\mathrm{n}\left(\mathrm{B}\right)-\mathrm{n}(\mathrm{A}\cap \mathrm{B})\\ =40+50-10=80\end{array}$

$\therefore$ Percentage reading either or both newspaper =80%. 

Hence, percentage reading neither newspaper =( 100 - 80)% = 20%