CX888
I don't know of any book that has loads of math exercises specifically tailored for aviation situations, though I have a book at home called Mental Math for Pilots by Ron McElroy. The title is self-explanatory, and it does give a good indication of what types of mental calculations are used by pilots on a regular basis. There aren't a lot of exercises inside but you can always create your own exercises after you've finished the ones in the book.
About the Kunming question... I will never know if they would have let me use the board or not. You can always ask them when the time comes, but be prepared if they say no. Though as some others have suggested, most interviewers are quite reasonable so it should be okay.