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

Total quantity of petrol consumed in 3 years
$$\eqalign{ & = \left( {\frac{{4000}}{{7.50}} + \frac{{4000}}{8} + \frac{{4000}}{{8.50}}} \right){\text{litres}} \cr & = 4000\left( {\frac{2}{{15}} + \frac{1}{8} + \frac{2}{{17}}} \right){\text{litres}} \cr & = {\frac{{76700}}{{51}}} {\text{ litres}} \cr & {\text{Total}}\,{\text{amount}}\,{\text{spent}} \cr & = Rs.\,\left( {3 \times 4000} \right) \cr & = Rs.\,12000 \cr & \therefore {\text{Average}}\,{\text{Cost}} \cr & = Rs.\, {\frac{{12000 \times 51}}{{76700}}} \cr & = Rs.\,\frac{{6120}}{{767}} \cr & = Rs.\,7.98 \cr} $$