其它
Ivan is puting 13 cakes into box A, box B and box C. It is known that there are odd number of cakes in both box A and box B. Could you tell whether the number of cakes in box C is even or odd number? 
Overseas Competition->Knowledge Point->Number Theory Modules->Odd and Even Numbers
Number of cakes in box C: 13 (odd number) - odd - odd  Number of odd character = 3  Therefore, the number of cakes in box C is odd. 