Well, yeah. Having the same sum, or having the same number of digits aren't really obvious properties of numbers. Similarly, both being prime and having the sa

Well, yeah. Having the same sum, or having the same number of digits aren't really obvious properties of numbers. Similarly, both being prime and having the same product aren't obvious properties either. Making the fundamental theorem of arithmetic non-obvious (or at least individual cases of it, but if the general case were obvious then the individual cases should also be obvious).