As many others have said, don't give a take home exam during a break, however 'normal' it is considered in your community.
Second, have clear guidelines on what is allowed and what is cheating. We never had take-home exams in school, and in uni every take-home exam was open book, open internet and open discussion. In the absence of any statement to the contrary, your students would also be justified in assuming so.
Asking someone to repeat the answer is fine, but it doesn't really prove anything - they might have simply forgotten all the formulae over their break.
The best option at this point would be to cancel that question and conduct future tests during class hours, under your supervision.