Premise: Margaret Hilda Thatcher , Baroness Thatcher , ( ; 13 October 1925 -- 8 April 2013 ) was a British stateswoman , who served as Prime Minister of the United Kingdom from 1979 to 1990 and as Leader of the Conservative Party from 1975 to 1990 . She was the longest-serving British prime minister of the 20th century , and the first woman to have held the office . As Prime Minister , she implemented policies that have come to be known as Thatcherism . She became Prime Minister after winning the 1979 general election . She resigned as Prime Minister and party leader in November 1990 , after Michael Heseltine launched a challenge to her leadership .
Hypothesis: Margaret Thatcher was a prime minister.
Label: ENTAILMENT