Shortcut Trick
Amount saved due to 30% reduction = 30% of Rs. 140
⇒ Savings = 0.30 × 140 = Rs. 42
This saved amount of Rs. 42 allows the customer to buy 5 more pens.
⇒ Reduced rate of Pen = Total savings ÷ Extra quantity = 42 ÷ 5 = Rs. 8.4
∴ The correct answer is Rs. 8.4.
Alternate Method
Given: Total expenditure = Rs. 140, Price reduction = 30%, Extra quantity = 5 pens.
Formula Used: Price × Consumption = Expenditure (If expenditure is constant, Price ∝ 1 / Consumption)
⇒ Let original price be 100 units. After 30% reduction, new price = 70 units.
⇒ Price Ratio (Old : New) = 100 : 70 = 10 : 7
⇒ Since expenditure is constant (Rs. 140), Consumption Ratio (Old : New) = 7 : 10
⇒ Increase in consumption = 10 − 7 = 3 units
⇒ According to question: 3 units = 5 pens
⇒ New consumption (10 units) = (5 ÷ 3) × 10 = 50/3 pens
⇒ Reduced rate = Total expenditure ÷ New consumption
⇒ Reduced rate = 140 ÷ (50/3) = (140 × 3) ÷ 50
⇒ Reduced rate = 420 ÷ 50 = Rs. 8.4
∴ The correct answer is Rs. 8.4.
Additional Information
Price-Consumption Relation
When the expenditure remains constant, the percentage increase in consumption due to a reduction in price is given by: [x / (100 − x)] × 100, where x is the reduction percentage.
Direct Formula for Reduced Price
Reduced Price = (Total Amount × Reduction %) ÷ Extra Quantity. Similarly, Original Price = Reduced Price × 100 ÷ (100 − Reduction %).
Percentage and Ratio Conversion
A reduction of 30% means the new value is 7/10 of the original value. The consumption must increase in the ratio of 10/7 to keep the product constant.