A car owner buys petrol at Rs 7.50, Rs 8.00 and Rs 8.50 per litre for three successive years, spending Rs 4000 in each year. Approximately what is the average cost per litre of petrol over the three-year period?

Introduction / Context:
This question requires calculating an average price per litre when the buyer spends the same amount of money at different per-litre prices in successive years. This is a classic application of the harmonic mean concept, because the quantity purchased varies inversely with price when the expenditure is fixed. However, we can solve it directly with basic arithmetic by working with total money spent and total litres bought.

Given Data / Assumptions:

• In year 1, petrol price = Rs 7.50 per litre and expenditure = Rs 4000.
• In year 2, petrol price = Rs 8.00 per litre and expenditure = Rs 4000.
• In year 3, petrol price = Rs 8.50 per litre and expenditure = Rs 4000.
• We must find the overall average cost per litre for the entire three-year period.

Concept / Approach:
The overall average cost per litre is defined as total money spent divided by total litres obtained over the three years. Since the car owner spends the same amount each year, the litres purchased in a year equals (yearly expenditure) / (price per litre). We compute the litres bought each year, sum them to get total litres, sum up the total expenditure, and then divide.

Step-by-Step Solution:
Total expenditure for three years = 4000 + 4000 + 4000 = Rs 12000.Year 1 price = Rs 7.50 per litre, so litres bought in year 1 = 4000 / 7.5.Year 2 price = Rs 8.00 per litre, so litres bought in year 2 = 4000 / 8.Year 3 price = Rs 8.50 per litre, so litres bought in year 3 = 4000 / 8.5.Compute these quantities: 4000 / 7.5 ≈ 533.33 litres, 4000 / 8 = 500 litres, 4000 / 8.5 ≈ 470.59 litres.Total litres bought ≈ 533.33 + 500 + 470.59 ≈ 1503.92 litres.Average cost per litre = total expenditure / total litres ≈ 12000 / 1503.92 ≈ Rs 7.98 per litre.

Verification / Alternative check:
We know that when equal amounts of money are spent at different prices, the average price is closer to the lower prices if more litres are bought at those prices. Here, more litres are bought at Rs 7.50 than at Rs 8.50, so the overall average must lie between 7.5 and 8.0. The computed value of about 7.98 fits naturally between these numbers and very close to 8, which is reasonable.

Why Other Options Are Wrong:
Option a, 6.23, is too low and would imply an unrealistically large number of litres purchased. Options c and d (8.97 and 9.89) are larger than any of the three yearly prices, which is impossible for an average price per litre when all purchases are at or below 8.50. Option e, 7.50, is just the first year's price and fails to account for higher prices in later years. Only 7.98 matches the computed average.

Common Pitfalls:
A frequent mistake is to simply average the three prices (7.5, 8, 8.5) arithmetically, which would give 8.0, ignoring the different quantities of petrol purchased at each price. The correct method is to use total expenditure divided by total litres. Careful calculation with division ensures that you arrive at the correct approximate figure.

Final Answer:
The approximate average cost per litre of petrol over the three years is Rs 7.98.