I bought a Mensa test book at the library book sale for a quarter (they were out of Sudoku books). According to the folks at Mensa, if you can answer this question, you can pass the admission test and become a Mensa member:
----------
ABCD, a four digit number, multiplied by 9 equals its mirror image, DCBA, a four digit number.
A, B, C, and D are all greater than or equal to 0 and less than or equal to 9.
A = ?
B = ?
C = ?
D = ?
Show your work.
----------
It's easy to guess the answer on a calculator, not so easy to come up with a coherent rational solution to the problem.
Good luck!