Given: Book seller buys notes from an institute at discount of 10% Student buys notes at a discount of 15%. Calculation: Let initial price of notes be 100 Price of study notes after discount of 10% = 100 × (90/100) = 90 Student buys study notes at a discount of 15% Price of study notes after further discount of 15% = 90 × (85/100) = 76.5 In order to restore original value of study notes, price to be increased = (100 - 76.5) = 23.5 on discounted price of 76.5 ∴ Percentage increase = (23.5/76.5) × 100 = 30.71% ≈ 31% Smart Trick: Let initial price of notes be 100 Price after discount = 100 × 0.9 × 0.85 = 76.5 In order to restore original value of study notes, price to be increased = (100 - 76.5) = 23.5 ∴ Percentage increase = (23.5/76.5) × 100 = 30.71% ≈ 31%