I have to confess, my brain stopped working for at least one minute as it was wondering even if this was a solvable question. Luckily, after 20 minutes thinking the solution emerges.
The problem is as follows:
The 23-Poker problem
You have 23 pokers on hand and you know 10 of them are face up but you close eye now. Find a way to seperate the pokers into two groups and make sure each group has same number of face-up pokers.
The solution is: put the pokers into two groups. One with 10 pokers and the other with 13. Now flip all pokers in the 10-group and you are set.
It is surprisingly easy. The trick here is you need to perform some operations on the pokers and many people are blocked here: sometimes we do need extra inspiration.
Sure, this simple problem can be extended. You can have n pokers with m face up. Now you need to seperate into a m-group and a n-m group. This problem also teaches us that we do have some way to ensure we can separate 2-type-items into two groups with one type equally distributed as long as we can transform one type to the other.