The biogas planner

A family wants to know what a gobar gas plant will give. You will write the sums for the gas, the energy and the size of the digester.

The idea. Each kg of dung gives some cubic metres of biogas. A share of the biogas is methane, which gives $35.8$ MJ for each m³. One kg of LPG gives $46$ MJ. The digester must hold the slurry of all the days that it stays in the tank.

Your tasks.

1. Finish biogas_m3. It gives the cubic metres of biogas each day.
2. Finish energy_mj. It gives the megajoules from the methane of that biogas.
3. Finish lpg_kg. It gives the kilograms of LPG with the same energy.
4. Finish digester_m3. It gives the volume of the digester, taking $1$ kg of dung as $1$ L of slurry.