Simplify the following text for a Grade 9 native English-speaking person.
Original: Robert Arthur Talbot Gascoyne-Cecil , 3rd Marquess of Salisbury , KG , GCVO , PC ( 3 February 1830 â `` 22 August 1903 ) , known as Lord Robert Cecil before 1865 and as Viscount Cranborne from 1865 until 1868 , was a British Conservative statesman and thrice Prime Minister , serving for a total of over 13 years .
Simplified: Robert Arthur Talbot Gascoyne-Cecil , 3rd Marquess of Salisbury ( February 3 , 1830 - August 22 , 1903 ) was a Prime Minister of the United Kingdom . He was known as Lord Robert Cecil before 1865 and as Viscount Cranborne from 1865 until 1868 .