Somebody can help me with this problem?

Question

I would like to know as I I decide this problem? A moved car the gasoline makes, on average 10 KM for liter, what it would correspond in a moved car the gas 15 KM for cubical meter of gas. It considers that the price of the liter of gasoline is of R$ 2,40 and the price for meter I cube of gas R$ 0,90. The cost for the conversion of the engine the gasoline for gas is of R$ 3,000,00. The necessary time so that a person who on average spends 120 liters of gasoline for month and that the gas converted the engine of its car at the beginning of the month pass to have advantage with the moved engine will be of, approximately:

a) 11 months

b) 14 months

Answer 1

120 liters gasoline x $2.40 = $288 for gasoline per month

120 liters gasoline x $2.40 x 10km/l = 2880 km per month.

2880km / 15km/cu m = 192 cu meters gas required to do same distance

192 x $0.90 = $172.80 for gas to travel same distance

288 - 172.80 = 115.20 saving per month

3000 / 115.20 = 26.04166667 months