Simplify the following text for a Grade 8 native English-speaking person.
Original: Sir Robert Gordon Menzies , ( 20 December 1894 â `` 15 May 1978 ) , Australian politician , was the 12th and longest-serving Prime Minister of Australia .
Simplified: Sir Robert Gordon Menzies ( 20 December 1894 â `` 14 May 1978 ) was the twelfth Prime Minister of Australia .