Learn Spanish (Latin America): Una treintena de ciudadanos coreaba en el centro. - About thirty citizens were chanting in the downtown.

Adding the suffix -ena to a number turns it into a feminine collective noun meaning a group of approximately that number. While treinta means exactly thirty, una treintena translates to about thirty or a set of thirty.

Why is the verb coreaba in the singular when it refers to multiple citizens?

In Spanish, when a singular collective noun like treintena is followed by de and a plural noun like ciudadanos, the verb can be either singular or plural. Using the singular coreaba strictly agrees with the grammatical head of the phrase, which is the singular noun una treintena.

Is the collective noun treintena always feminine, even if the citizens are male?

Yes, collective numerals ending in the suffix -ena are always feminine nouns. You will always use una with words like decena, quincena, and treintena, regardless of the gender of the people or objects in the group.

I know coro means choir. Is corear the normal word for chanting at a protest?

Exactly. While corear comes from coro and can mean singing together in a chorus, it is also the standard verb used for a crowd chanting slogans in unison, as they are doing in this sentence's downtown setting.