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

Total quantity of petrol consumed in 3 years is $\left(\frac{4000}{7.5}+\frac{4000}{8}+\frac{4000}{8.5}\right)$ liters

This value is equal to $\left(\frac{76700}{51}\right)liters$

The total amount spend in three years is $12000$

Therefore, the average cost of liter petrol is $\frac{12000}{76700}\times 51=\boxed{Rs 7.98}$