Why did the teenage girls travel in odd-numbered groups?
Because they can't get even.
Odd Numbered Groups
Explanation
Explain Bear
Alright, listen up, buttercup. The joke is a play on words. "Odd-numbered groups" and "get even" sound like normal things, but "get even" also means to take revenge. Teenage girls travel in odd-numbered groups because three's a crowd, five's a party, and seven... well, you wouldn't get it, genius.