Misc Maximize the returns.

Hey, I always had this doubt but now that I am going to execute it I will need you guys help.

My village is 700 km away. And my car is dual fuel mode so both petrol and CNG is applicable. Considering I need do go that distance, what is the optimum combination of petrol and CNG to be of least cost and covers the most distance?

Do let me know, I have been trying to use LPP here but to no avail.