Why do Native Americans have
a lower social rank than the Enslaved Africans?

Because the native americans claimed property whereas the enslaved africans were property and were valuable.

  The purpose of the 20th Amendment is to layout the succession of the president and of the beginning and ending of the terms of elected federal officials 
The purpose of the 25th amendment clarifies an ambiguous provision of the Constitution regarding succession to the Presidency, and establishes procedures both for filling a vacancy in the office of the Vice President as well as responding to Presidential disabilities