I know that the most populous country in Africa is Nigeria (source: The World Factbook).
Using this information, I determined when the constitution was adopted in Nigeria:
Constitution:
history: several previous; latest adopted 5 May 1999, effective 29 May 1999
amendments: proposed by the National Assembly; passage requires at least two-thirds majority vote of both houses and approval by the Houses of Assembly of at least two-thirds of the states; amendments to constitutional articles on the creation of a new state, fundamental constitutional rights, or constitution-amending procedures requires at least four-fifths majority vote by both houses of the National Assembly and approval by the Houses of Assembly in at least two-thirds of the states; passage of amendments limited to the creation of a new state require at least two-thirds majority by the proposing National Assembly house and approval by the Houses of Assembly in two-thirds of the states; amended several times, last in 2012 (2017)
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Source: The World Factbook