If you have a device that has an energy efficiency of 0.4 and you put in 100 units of energy, how many units of useful energy will you get out of it?

If its 0.4%, the answer will be 0.4

or else it will be 40. 

The answer is water molecules are split.cellular respiration and photosynthesis are complement  reactions to each other. these are reverse reactions. in cellular respiration glucose and oxygen yield water and carbon dioxide. so, when respiration begins,  glucose molecules are split. in the opposite reaction - in photosynthesis, carbon dioxide and water yield glucose and oxygen. thus,  when respiration begins, water molecules are split.