Subject-verb agreement while talking about percent [duplicate]
Solution 1:
20% of the participants believes/believe that the world is flat
The noun "percent" (or just %) belongs with the number-transparent nouns like "lot" and "number", where the number of the whole noun phrase depends not on the head, but on the number of the noun that is complement to the preposition "of".
In this case, the complement of "of" is the plural noun "participants", so the NP requires the plural verb "believe".