A car owner buys petrol at Rs.7.50, Rs. 8 and Rs. 8.50 per litre for three successive years. What approximately is the average cost per litre of petrol if he spends Rs. 4000 each year?

Given

• Year 1 price = Rs. 7.50/L; Year 2 = Rs. 8.00/L; Year 3 = Rs. 8.50/L
• He spends Rs. 4,000 each year (equal money, different litres)

Concept: When quantities differ, the average price is a weighted average by litres, not a simple mean of prices.

Step 1: Litres bought each year
L₁ = 4000 / 7.50 = 533.33… LL₂ = 4000 / 8.00 = 500.00 LL₃ = 4000 / 8.50 ≈ 470.59 LTotal litres L ≈ 533.33 + 500.00 + 470.59 = 1,503.92 L

Step 2: Average cost per litre
Total spend over 3 years = 3 × 4000 = Rs. 12,000Average price = Total spend ÷ Total litres≈ 12,000 ÷ 1,503.92 ≈ Rs. 7.98/L, i.e., about Rs. 8.00 per litre (approx.)

Why not just average 7.50, 8.00, 8.50?

• Because he buys more litres at the lower price and fewer litres at the higher price; litres-weighted averaging reflects this.

Final Answer
Approximate average cost ≈ Rs. 8.00 per litre.