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The electric charge on baryons (e.g. protons) and leptons (e.g. electrons) are the same in magnitude. Baryons are made of quarks but Leptons are not. So where is the connection which makes the electric charge EXACTLY the same in magnitude? If these charges were not equal the whole atomic structure would be different, and atoms possibly unstable. Put more crudely, how does an electron "know" what the charge is on a proton if they have no structure in common?