A person sells two articles at the same price. On one he gain 20% and losses 40% on the other. Find his gain or loss percent on the whole.

$\begin{array}{l}LeteachSP=100\\ by selling 1^{st}\text{\hspace{0.17em}article he gets \hspace{0.17em}}20\%profit\\ 120\%OF CP=100\\ cp of first article=\frac{250}{3}\\ by selling 2^{nd}\text{\hspace{0.17em}article \hspace{0.17em}} \mathrm{he} \mathrm{gets} 40\%loss\\ 60\%OF CP=100\\ cp=\frac{500}{3}\\ Total\text{\hspace{0.17em} CP=}\frac{250}{3}+\frac{500}{3}=\frac{750}{3}=250\\ TotalCP=250\\ TotalSP\to 100+100=200\\ Loss=250-200=50\\ loss\%=\frac{50}{250}\times 100\%=20\%loss\end{array}$