As provided in the question, heat supplying natural gas furnace operates at 60% efficiency. It means that out of 100 units of heat energy produced by the furnace, only 60 units of heat can be transferred.
Thereby as the demand of the heat energy is 120 MBtu, amount of gas that needs to be purchased equals-
∴ 60% of (cubic metres of gas that needs to be purchased) = 120 MBtu
⇒ (60/100) * gas = 120
⇒ Gas= 120*100/60
∴ Cubic metres of gas that needs to be purchased= 200MBtu
Generally, rings form from moons, asteroids, or comets that have disintegrated due to a collision or because they got too close to their planet (Roche Limit). ... Most of the debris, however, will not have enough energy to overcome the planet's gravity and will remain in orbit around the planet.