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?

1. Rs. 8.50
2. Rs. 9
3. Rs. 8
4. 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.