By what percent above the cost price must a dealer mark his goods so that, after allowing a discount of 20% on the marked price, he still makes a profit of 25%?

Take CP = ₹100, so the selling price must be ₹125. The discount leaves 80% of the marked price, so MP × 0.80 = 125 → MP = ₹156.25 — a mark-up of 56.25%. (Adding 20 + 25 = 45% forgets that the 20% is taken off the larger marked price, not off the cost.)