Here's how she does it:
The game asks you to select any two digit number (example 25). Next, you are to subtract both the first and second digits from your number (example: 25 – 2 – 5 = 18). Then you are to select a gift from a grid of all possible answers. In the final page, Regifting Robin correctly guesses your gift. So how does this nifty trick work?
The equation can be represented as: (10*x + y) - x - y = 9*x
Using our example, x = 2 and y = 5, so that’s (10*2 + 5) – 2 – 5 = 9*2 = 18.
In the gift grid, each 9*x box has the same gift, e.g., the boxes numbered 9, 18, 27, 36, etc. all are marked “Bath Salts.”
The selected gift is changed each time to make the trick seem more real.
I found this on a yahoo search.
http://www.devtopics.com/regifting-robin/