In the window of my pocket calculator, I can see a six-figure number, abcdef, with distinct digits between 1 and 9. A close inspection of the digits reveals that abcdef, cdef, and def are all prime numbers.

If I turn the calculator upside-down, I see that uvwxyz and wxyz are also prime numbers.

Which number is displayed in the window of my pocket calculator and what is the upside-down version of it?

Full credit for this problem goes to Ajit Athle who has very kindly given his permission for the puzzle to be posted here.

I found 182659 but then realized that when the "6" and "9" flip they are no longer "6" and "9" but are now "9" and "6".

Back to the drawing board.

Posted by Leming on 2006-12-15 10:23:28