you can see the statistical consumption cost ratio.
A LPG cylinder if full with up to 50liters of gas and costs around 55Rs. per liter.
the total cost would be 55x50=2750
the mileage given is 600 km therefore 2750 divided by 600 = 4.60 Rs. approximately.
better than CNG.
Full tank with 9 Kg gas 79x9= 711
mileage per tank 135 km so 5.30Rs Approximately.
But keeping in mind that to complete a 600 km journey you will be needed to fill the cylinder at least 5 times. so 711x5= 3500 Rs. approximately.
You can clearly see the difference that what fuel is economical.
And to take other factors into account that the LPG does not harm the engine and does not cut its peak performance.. whereas on CNG you get both.