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

Rs. 7.98
Rs. 8
Rs. 8.50
Rs. 9
Explanation:
Total quantity of petrol
consumed in 3 years
=$ \left(\dfrac{4000}{7.50} +\dfrac{4000}{8} +\dfrac{4000}{8.50} \right) $ litres
= 4000$ \left(\dfrac{2}{15} +\dfrac{1}{8} +\dfrac{2}{17} \right) $ litres
=$ \left(\dfrac{76700}{51} \right) $ litres

Total amount spent = Rs. $\left(3 \times 4000\right)$ = Rs. 12000.

$\therefore$ Average cost = Rs.$ \left(\dfrac{12000 \times 51}{76700} \right) $= Rs.$ \dfrac{6120}{767} $   = Rs. 7.98
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