A dishonest dealer sells the goods at 20% loss on cost price but uses 15% less weight. What is his percentage profit or loss ?

$$\eqalign{ & {\text{Gain/loss }}\% \cr & = \left\{ {\left( {\frac{{y - x}}{{100 - y}}} \right) \times 100} \right\}\% \cr & = \left\{ {\left( {\frac{{15 - 20}}{{100 - 15}}} \right) \times 100} \right\}\% \cr & = \left( {\frac{{ - 5}}{{85}} \times 100} \right)\% \cr & = - \frac{{100}}{{17}}\% \cr & = - 5\frac{{15}}{{17}}\% \cr} $$
Since it is -ve, hence it is a loss.