<span>Mass defect is the difference between predicted and actual experimentally determined mass. The predicted mass of a sample is found by adding the mass of the neutrons and protons (to be accurate, electrons as well) within that sample, and we know what the individual masses of these particles are so multiplication isn't a problem. This mass is usually higher than what is measured experimentally because some of this mass turns to binding energy with the relationship e is roughly equal to m(c)^2.
If you are asking how we possibly add the masses of all those atoms of the Helium and Hydrogen we experiment with, we do it using the formula (number of atoms * number of protons per atom) + (number of atoms * number of neutrons per atom) + (number of atoms * number of electrons per atom). The number of atoms is found using the total mass of the sample alongside some experimental data.</span><span>
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The answer is photosynthesis.
