The calculation assumes a population of 210 people: 10 librarians and 200 farmers. With 40% of librarians fitting the description (yielding 4 matching librarians) and 10% of farmers fitting it (yielding 20 matching farmers), there are 24 total matches.
Conditions: Population consists only of librarians and farmers with a 1:20 ratio; Likelihoods are stipulated as 40% for librarians and 10% for farmers; Conditioning requires