Introduction
The process of photosynthesis involves the fixation of atmospheric carbon dioxide (CO2) into organic molecules such as sugars. The first stable product formed during this process is a three-carbon molecule called phosphoglyceric acid (PGA). Therefore, to determine the number of carbon atoms in a molecule that has fixed CO2 and produced PGA, we need to understand the process of carbon fixation and PGA synthesis.

The Process of Carbon Fixation
During photosynthesis, CO2 enters the chloroplasts of plant cells and is converted into an organic molecule through a series of enzyme-catalyzed reactions. The first step of this process is the fixation of CO2 into a five-carbon molecule called ribulose bisphosphate (RuBP), which is catalyzed by the enzyme Rubisco. This results in the formation of an unstable six-carbon intermediate, which quickly breaks down into two molecules of PGA.

The Synthesis of PGA
PGA is a three-carbon molecule that is produced during the light-independent reactions of photosynthesis. After CO2 is fixed into RuBP, the resulting six-carbon intermediate quickly breaks down into two molecules of PGA. These molecules are then used to synthesize other organic molecules such as glucose through a series of enzyme-catalyzed reactions.

Calculating the Number of Carbon Atoms
Since each molecule of PGA contains three carbon atoms, the number of carbon atoms in a molecule that has fixed CO2 and produced PGA can be determined by dividing the total number of carbon atoms in the organic molecule by three. For example, if the organic molecule contains nine carbon atoms, it would have produced three molecules of PGA.

Conclusion
In summary, the number of carbon atoms in a molecule that has fixed CO2 and produced PGA can be determined by understanding the process of carbon fixation and PGA synthesis. Each molecule of PGA contains three carbon atoms, so the total number of carbon atoms in the organic molecule can be divided by three to calculate the number of PGA molecules produced.