If the price of petrol increases by 30% and Babu intends to spend only an…

Concept: Expenditure = Price × Quantity. If price rises by p% while the amount one is willing to spend rises by only b% (with b < p), the quantity consumed must fall. The required percentage reduction in quantity equals 1 − (100 + b)/(100 + p), expressed as a percentage.

Application: Let the original price of petrol be Rs. 100 and the original expenditure also be Rs. 100 (so the original quantity corresponds to 1 unit of spending).

1. New price after a 30% increase = 100 × (130/100) = Rs. 130.

2. Babu allows only a 10% increase in expenditure, so his new spending = 100 × (110/100) = Rs. 110.

3. New quantity bought = new expenditure ÷ new price = 110/130 = 11/13 of the original quantity.

4. Reduction in quantity = 1 − 11/13 = 2/13 of the original quantity.

5. Percentage reduction = (2/13) × 100 = 200/13 ≈ 15.38%.

Cross-check: Using the direct ratio form, percentage reduction = (p − b)/(100 + p) × 100 = (30 − 10)/130 × 100 = 20/130 × 100 ≈ 15.38%, which matches the step-by-step result above.

Answer: Babu must reduce his petrol consumption by approximately 15.38%.