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A car owner buys petrol at Rs. 7.50 Rs. 8.00 and Rs. 8.50 per litre for the 3 successive years. If he spends Rs. 4000 each year, then the average cost per litre of petrol is

$\begin {array} {1 1} (A)\;Rs. 8 & \quad (B)\;Rs.8.25 \\ (C)\;Rs. 7.98 & \quad (D)\;None \: of \: these \end {array}$

Petrol bought in the first year = $\large\frac{4000}{7.50}$ litres
Petrol bought in the second year = $\large\frac{4000}{8}$ litres
Petrol bought in third year = $\large\frac{4000}{8.50}$ litres
Total money = Rs. 12,000
Average cost per litre = $\large\frac{120000}{\Large\frac{4000}{7.50}+\Large\frac{4000}{8}+\Large\frac{4000}{8.50}}$
$= 7.98$
Ans : (C)