A trader gains 25% by selling an article with 20% discount on its marked price. If the cost price of the article increased by 30%, then how much discount (in %) should he offer on the same marked price to gain 15% of profit?

$$\eqalign{ & \frac{{{\text{Cost price}}}}{{{\text{Marked price}}}} = \frac{{100 - {\text{Discount}}}}{{100 + {\text{Profit}}}} \cr & = \frac{{100 - 20}}{{100 + 25}} \cr & = \frac{{80}}{{125}} \cr & {\text{New cost price}} = 80 \times \frac{{130}}{{100}} = 104 \cr & {\text{New selling price}} = 104 \times \frac{{115}}{{100}} = 119.6 \cr & {\text{Discount}} = 125 - 119.6 = 5.4 \cr & {\text{Discount }}\% = \frac{{5.4}}{{125}} \times 100 = 4.32\% \cr} $$