Here Enoch is a descendant of Cain.
Gen 5 states:
3 When Adam had lived 130 years, he fathered a son in his own likeness, according to his image, and named him Seth. 4 Then the days of Adam after he fathered Seth were eight hundred years, and he fathered other sons and daughters. 5 So all the days that Adam lived were 930 years, and he died.
6 Now Seth lived 105 years, and fathered Enosh. 7 Then Seth lived 807 years after he fathered Enosh, and he fathered other sons and daughters. 8 So all the days of Seth were 912 years, and he died.
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18 Now Jared lived 162 years, and fathered Enoch. 19 Then Jared lived eight hundred years after he fathered Enoch, and he fathered other sons and daughters. 20 So all the days of Jared were 962 years, and he died.
21 Now Enoch lived sixty-five years, and fathered Methuselah. 22 Then Enoch walked with God three hundred years after he fathered Methuselah, and he fathered other sons and daughters. 23 So all the days of Enoch were 365 years. 24 Enoch walked with God; and he was not, for God took him.
Here Enoch is a descendant of Seth.
What's going on?