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Answer: 2, 4 and 6

Jenny must know for sure that Tom's cards are either both even or both odd.

1  2  3  4  5  6  7  
4 odd numbers and 3 even numbers
So Jenny must have taken the 3 evens and left the 4 odds



This problem is taken from the UKMT Mathematical Challenges.
You can find more short problems, arranged by curriculum topic, in our short problems collection.