My Scooty gives an average of 40 kmpl of petrol. But after recent filling at the new petrol pump, its average dropped to 38 kmpl. I investigated and found out that it was due to adulterated petrol. Petrol pimps add kerosene, which is $$\frac{2}{3}$$  cheaper than petrol, to increase their profits. Kerosene generates excessive smoke and knocking and gives an average of 18 km per 900 ml. If I paid Rs. 30 for a litre of petrol, What was the additional amount the pump-owner was making?
a) Rs. 1.75
b) Rs. 1.80
c) Rs. 2
d) Rs. 2.30

Answer: c
Explanation: Let x ml of kerosene be there in 1 litre mixture.
Then, quantity of petrol in 1 litre mixture = (1000 – x) ml
$$ \frac{{40}}{{1000}}\left( {1000 – x} \right)$$   $$ + \frac{{18}}{{900}}x$$   = 38
$$\eqalign{ & \frac{x}{{25}} – \frac{x}{{50}} = 2 \cr & \frac{x}{{50}} = 2 \cr & x = 100 \cr} $$
So, 1 litre mixture has 900 ml petrol and 100 ml kerosene.
Cost of 1 litre petrol = Rs. 30
Cost of 1 litre kerosene
= Rs. $$\left[ {\left( {1 – \frac{2}{3}} \right) \times 30} \right]$$
= Rs. 10
Coast of 1 litre mixture
= Rs. $$\left( {\frac{{30}}{{1000}} \times 900 + \frac{{10}}{{1000}} \times 100} \right)$$
= Rs. 28
Additional amount earned by pump-owner
= Rs. (30 – 28)
= Rs. 2