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?

- Rs. 8.50
- Rs. 9
- Rs. 8
- Rs. 7.98

**Solution**

Average cost per litre of petrol = Total amount / Total quantity of petrol

Re. 4000 is spent each year, so total amount spent = 3 * Rs. 4000 = Rs. 12,000

Total quantity of petrol consumed in 3 years = (4000/7.50) + (4000/8) + (4000/8.50) litres

= 533.3 + 500 + 470.6 = 1505

Average cost = Total amount / Total quantity

= 12000/1504 = 7.98

To find quickly, you can take cube root of (7.50 * 8 * 8.50), and it will be slightly less than 8.

**The correct option is D.**