Hello
I'd build on some of the answers provided below. Think about why does any interviewer ask this question. They are testing your general knowledge and your ability to develop a structured response to a highly ambiguous question.
1) What is the world's current population? About 7.7 billion and ask if you can round it up to 8 billion people.
2) The hemispheres are separated by the equator which passes through Ecuador in South America splitting the Americas, through (almost) middle of Africa and through the Indian Ocean. That means majority of South America, slightly less than half of Africa, parts of Asia (Indonesia, PNG, a few islands etc), Australia and NZ are the Southern Hemisphere.
4) Now try to assign South Hemipshere a % of the 8 billion. Explain the reason you've chosen this route is the 'elimination' technique rather than a bottoms-up technique of building the Nothern Hemisphere calculation. You could try estimating this population or ask if you can use an average of population per country based on 8 billion divided by 180 countries. I'd personally like to see the former. A great way to approach this would be AUS + NZ + Indonesia + Brazil is almost USA + Mexico + Canada, so ~500 million. So now you have estimated the 'Elephants' of the Southern Hemisphere.
5) Ask what data points can be used now for remainder of South America and for the bottom half of Africa. You can say based on your understanding of the geography the land size and density is similar to it should be at least another 500 million, to a total of 1 billion.
6) 1 billion / 8 billion = ~12%. So 88% lives in Northern Hemisphere.