Since he use over 2800 pea plants with the same experiment it made his actual results approach the results predicted by probability 
        
             
        
        
        
The ''x'' in the word equation for photosynthesis is ATP
<u>Explanation:</u>

Oxygen + glucose carbon dioxide + water + x
A chemical reaction happens in this photosynthesis where the atom arrangements are changed to different arrangement to form a new substance.  When during the time of photosynthesis, the energy from the Sun is stored in the form of chemical energy. 
Here the X is ATP, the Adenosine triphosphate which provides energy for the plants. ATP releases small amount of energy through hydrolysis.