A shopkeeper has been selling an article at a profit of 20%. The cost price now falls by 10% while he keeps the selling price unchanged. His profit percent becomes:

Take the old cost as ₹100, so the selling price is ₹120. The new cost is ₹90 and the selling price is still ₹120, so the profit is ₹30 on ₹90 = 33.33%. (Adding the 10% saved to the old 20% gives 30% and forgets that the base has shrunk to 90; keeping the profit at ₹20 while switching the base to 90 gives the other trap, 22.22%.)