UPSC CAPF Assistant Commandant Model Paper 7

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%