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Question

A car owner buys petrol at Rs. 7.50, Rs. 8.00 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?


A
Rs. 7.98
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B
Rs. 8
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C
Rs. 8.50
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D
Rs. 9
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Solution

The correct option is D Rs. $$7.98$$
Given that he spends Rs. $$4000$$ each year 

The cost of the petrol in the first year is Rs. $$7.50$$ per litre

Number of litres, he bought in the first year is $$\dfrac{4000}{7.5}$$

The cost of the petrol in the second year is Rs. $$8.00$$ per litre

Number of litres, he bought in the second year is $$\dfrac{4000}{8}$$

The cost of the petrol in the third year is Rs. $$8.50$$ per litre

Number of litres, he bought in the third year is $$\dfrac{4000}{8.5}$$

Average cost $$=\dfrac{4000+4000+4000}{\dfrac{4000}{7.5}+\dfrac{4000}{8}+\dfrac{4000}{8.5}}$$

$$=\dfrac{1+1+1}{\dfrac{1}{7.5}+\dfrac{1}{8}+\dfrac{1}{8.5}}$$

$$=\dfrac{3\times 7.5\times 8\times 8.5}{(8)(8.5)+(7.5)(8.5)+(7.5)(8)}$$

$$=\dfrac{1530}{191.75}=7.98$$

Therefore the average cost per litre of pertrol is Rs. $$7.98$$

Mathematics

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