Analysis of results

Key Findings:
Amount of coke does NOT affect the voltage produced.
Salt added to the coke DECREASES the voltage produced.

This is why coke produces a voltage:
The copper holds onto its atoms more strongly than the zinc does. The zinc strip is therefore more negative than the copper strip, and the electrons flow from the zinc to the copper. When the forces are eventually balanced, the copper strip ends up with more electrons than the zinc strip. The zinc strip now has fewer electrons, and it cannot attract the zinc ions back to the strip. If our battery just had water in it, not much more would happen. But our Coca-Cola battery has water plus phosphoric acid. An acid is something that has an easily detached hydrogen ion. Hydrogen ions are positive, and the remaining part of the acid becomes negative when it loses the hydrogen ion. In our battery, the remaining part is the phosphate ion. So what happens when all of those positively charged zinc ions bump into those negatively charged phosphate ions? The phosphate ion is more strongly attracted to the zinc ion than to the hydrogen ion. The positively charged hydrogen ion is attracted to the copper strip, because the copper strip has the extra electrons, and is thus negative. The hydrogen ions attract the electrons from the copper, and become neutral hydrogen atoms. These join up in pairs to become hydrogen molecules, and form bubbles on the copper strip. Eventually the bubbles become big enough to float up to the surface and leave the system entirely. Now the copper strip no longer has the extra electrons. It attracts more from the zinc strip through the connecting wire, as it did when we first connected the wire. The copper ions next to the copper strip are not as attracted to the strip as they were before. The hydrogen ions keep taking the electrons that attracted the copper ions. So those ions are free to move through the liquid. At the zinc strip, zinc ions are being removed, leaving extra electrons. Some of those electrons travel through the wire to the copper strip. But some of them encounter the copper ions that happen to bump into the zinc strip. Those ions grab the electrons, and become copper atoms. We can see those atoms build up on the zinc strip. They look like a black film, because the oxygen in the water combines with the copper to form black copper oxide. Eventually, all of the zinc is eaten up, and the copper and copper oxide falls into a pile beneath where the zinc strip used to be. The battery is now dead, and no more electrons flow through the wire. If there was not a lot of acid in the water, it may be the first thing to be used up, and the battery may die while there is still some zinc left on the zinc strip. (FIELD, 2015)

Notice from the explanation, amount of coke does not affect to voltage produced, but very likely affects the length or duration the voltage can be kept

This is why salt decreases the voltage:
What happens exactly when salt is dropped into coke? Dropping salt in soda causes the salt to dissolve and subsequently reduce the availability of the soda's solvent to dissolve the carbon dioxide, which is due to the increase in solutes causing a change in the balance of the soda's chemistry, also causing the soda to release some of its carbon dioxide. The salt falling into the soda allows for the formation of nucleation sites for carbon dioxide bubbles. These bubbles transition to the surface and cause the bubbling effect you notice after pouring salt into soda. The salt dissolving into the soda causes a change in the pH balance of the solution. In return, the pH shift causes a decrease in the solubility of the carbon dioxide, thus releasing CO2 gas, or the bubbles you see when salt is combined with soda. How does this decrease the voltage produced? As the battery heavily relies on the acid in the coke for it to work, the pH shift caused by the salt will cause less acid to produce voltage. This in turn leads to lower readings on the voltmeter. (KILHEFNER, 2015)

Therefore, our hypothesis is NOT fully correct.
Our hypothesis was: The greater the amount of coke, and the lesser the amount of salt added to the coke, the more the voltage is produced.

HOWEVER, the amount of coke does not affect the voltage produced.
The correct conclusion should be: Coke does not affect the voltage produced, and the lesser the amount of salt added to the coke, the more the voltage is produced.