Explain the difference between the 20th and 25th amendments.

  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

20th Amendment (1933) Congress begins on January 30th; President starts on January 20th
"Lame-duck" Amendment
25th Amendment (1967) (1) Succession of VP if president dies or become incapable to do his job.(2) if there is no VP, president must appoint one, and congress must approve