A car owner buys petrol at the rate ₹ 17, ₹ 19 and ₹ 20 per litre, respectively for three consecutive years. Compute the average cost per litre, if he spends ₹ 6,460 per year for the three consecutive years.

It is given, the average cost per year for the three given years = ₹ 6,460

Now, let's say in each year the cost of petrol had been ₹ 6,460

So, amount of petrol bought in first year = $$\frac{6460}{17}=380$$ litre

amount of petrol bought in second year = $$\frac{6460}{19}=340$$ litre

amount of petrol bought in third year = $$\frac{6460}{20}=323$$ litre

So, total amount of petrol bought in three years = $$380+340+323=1043$$ litre

Now total cost over the three years = $$3\times\ 6460=19380$$

So, average cost per litre = $$\frac{19380}{1043}=₹18.58$$