Answer:
Average cost per litre of diesel = rs 8.00
Step-by-step explanation:
Amount spent on diesel each year = rs 4000
Price of a litre of diesel in the first year = rs 7.50
Price of a litre of diesel in the second year = rs 8.00
Price of a litre of diesel in the third year = rs 8.50
Average cost per litre of diesel = 
where n is the number of years.
Average cost per litre of diesel = 
= 
= 8
Average cost per litre of diesel is rs 8.00
So that he buys an average of 500 (i.e
) litres every year.