The 20th Century is 1901-2000, so the VP had to become president during that span. Four VPs during that span became Presidents due to the death of the President: Theodore Roosevelt (as of Sept. 1901), Calvin Coolidge (as of August 1923), Harry Truman (as of April 1945), and Lyndon Johnson (as of November 1963). One VP became President due to the resignation of the President: Gerald Ford (as of August 1974). And two VPs were elected President in their own right: Richard Nixon (1969) and George H.W. Bush (1989). So 4+1+2, or 7, VPs made the jump to president during the 20th Century.