Very easy to solve, just like it is easy to solve many other ones once you know the tricks.

I recommend this book: https://www.amazon.com/Secrets-Mental-Math-Mathemagicians-Ca...

I'd vote for this: http://www.amazon.com/Secrets-Mental-Math-Mathemagicians-Cal... and this: http://en.wikibooks.org/wiki/Mental_Math

It's interesting that the book cited in the submitted article

http://www.amazon.com/Secrets-Mental-Math-Mathemagicians-Cal...

(this link is to the United States Amazon site, while the submitted article link was to amazon.co.uk)

is written mostly by a professional mathematician, but with an introduction by an author who is a historian by higher education. Michael Shermer writes a number of interesting books,

http://www.amazon.com/Michael-Shermer/e/B001H6MCNY/

of which my favorite is Why People Believe Weird Things: Pseudoscience, Superstition, and Other Confusions of Our Time.

http://www.amazon.com/People-Believe-Weird-Things-Pseudoscie...