Do not remember.

- Five Card Trick by Colm Mulcahy in MAA Horizons, 10 February 2003.
- Interesting variations of the puzzle are in Card Colm by Colm Mulcahy in MAA Horizons, Feb 2005.
- Check out N-Card Tricks by Hang Chen and Curtis Cooper, The College Mathematics Journal, Volume 40, Number 3, May 2009 , pp. 196-203(8).
- A related puzzle at MathOverflow.Net.

Find the measure of angle "a" in the diagram.

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