Simplify the following text for a Grade 8 native English-speaking person.
Original: Sir John `` Black Jack '' McEwen , GCMG , CH ( 29 March 1900 â `` 20 November 1980 ) , was an Australian politician and the 18th Prime Minister of Australia .
Simplified: Sir John McEwen ( 29 March 1900 â `` 20 November 1980 ) was the 18th Prime Minister of Australia .