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