If both the President and the Vice President can no longer serve, who becomes President?

Why this is the answer

If the President and the Vice President both become unable to serve, the Speaker of the House of Representatives is next in line. The rule has moved around over the years. Early in the country's history the law put the president pro tempore of the Senate next after the Vice President, and later the Secretary of State held that place. The Presidential Succession Act of 1947 returned the job to a congressional leader, and the 25th Amendment, ratified in 1967, set out the procedures for presidential and vice presidential succession. Memory hook: after the President and the Vice President, the line runs to the leader of the House, not to a Cabinet officer.