The cardinal number that is the product of four and ten; 40.
From Old English 'feowertig', literally meaning 'four tens'. The spelling changed from 'fourty' to 'forty' in the 17th century, making it the only number between twenty and one hundred that doesn't contain its base number's spelling.
Forty is the only number whose letters appear in alphabetical order when spelled out in English! It also has deep cultural significance - appearing 146 times in the Bible as a period of testing or trial, like the 40 days of rain during Noah's flood.
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