Conversation so far:
how many ethnic groups did we have in nigeria
Nigeria has 250 ethnic groups.
what's the biggest ethnic group in nigeria
the Hausa are the biggest ethnic group in Nigeria
what's the population of Hausa in nigeria
there are 67 million Hausa in Nigeria
what percentage of nigeria is hausa
the hausa make up 25% of the population in nigeria

Question: what's the second biggest
Self-contained rewrite: what's the second biggest population in hausa