It is the substance that is totally consumed when the chemical reaction is complete.
The energy containing electron transporters of FADH2 are not produced during glycolysis.
Chemical bonds are broken to supply us with energy. Without bonds, and our ability to break them, the food we eat would not supply us with energy.
The answer is 3. As 5 * 3 = 15.
Is there any other equations I may be able to help you with? :)
