Learn Swedish: Statistiken visar hundra personer, varav nittio är studenter. - The statistics show a hundred people, of which ninety are students.

Varav is a formal relative adverb meaning 'out of which' or 'of whom.' It is highly idiomatic in Swedish for introducing a subset of a group you just mentioned (here, the hundred people). While av vilka is grammatically correct, varav is the standard, natural choice for presenting subsets in statistics and formal writing.

Why is Statistiken in the singular form when the English translation says 'statistics'?

In Swedish, statistik is an uncountable noun, meaning it is always singular. That is why it takes the singular definite ending -en (statistiken) instead of a plural ending. While English treats 'statistics' (the data) as plural, Swedish treats the collection of data as a single mass.

Why doesn't the sentence say ett hundra personer for 'a hundred people'?

When saying 'a hundred' or 'a thousand' in Swedish, you generally drop the word ett (one/a) and just say hundra or tusen. You only use ett hundra if you need to strongly emphasize that it is exactly one hundred and not two or three.

Since varav starts a new clause, what happens to the word order if I want to add a word like inte (not)?