Correct Answer - Option 4 : 5∶9
CNG car gives an average that is 20% higher than petrol car.
Also, petrol costs 50% of its price higher than CNG.
Simple concept of Percentage and Ratio.
Let the Mr. Y's petrol car covers a distance of 100 km in per liter.
Then, Mr. X's CNG car covers a distance of 120 km in per liter.
Let the cost of CNG per litre = Rs. 100
then, the cost of petrol per liter = Rs. 150
now, cost per liter of Mr. Y's car = 100/150 = 2/3
Also, cost per liter of Mr. X's car = 120/100 = 6/5
Required ratio = 2/3 ∶ 6/5 = 5 ∶ 9
∴ The ratio of cost per kilometer of Mr. Y's car to Mr. X's car is 5∶9.