Complex sentence: In the government of Arvid Posse in 1880, he became Minister without Portfolio and, in 1881 Minister of Finance, which he was for three years before he became Prime Minister at the age of 40.
Split into simpler sentences: In the government of Arvid Posse in 1880, he became Minister without Portfolio and, in 1881 Minister of Finance. He stayed on in this office in three years before he became Prime Minister at the age of 40.