What is the best way to quickly improve at mental math? [closed]

I am doing a interview on Monday that is likely to be based on performing basic computations very quickly (45% of 120, closest integer to square root of 1789, etc). I am wondering what is the best way to improve rapidly at adding and multiplying numbers quickly in my head. Specifically, any books I can get for Kindle or online resources for testing out answers would be very helpful. What resource would you recommend?

Thanks very much for your help.

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This website is intended for research-level mathematics. The FAQ lists a number of other websites that are more appropriate for elementary questions. – Andy Putman Dec 24 2010 at 21:42
en.wikipedia.org/wiki/Alberto_Coto_Garc%C3%ADa – Andres Caicedo Dec 24 2010 at 21:43
I remember years ago reading a book about savants. One went into a great deal of detail about the mental pictures he created for himself to help his computations. There were rivers and trees in them. A Colombian calculator I saw once while in high school said he would give names to numbers, so he would remember stories, that are easier than strings of numbers. So, a sequence such as 3.1415926... would become a story of a bear in the woods hunting for food. – Andres Caicedo Dec 24 2010 at 21:49
One of my colleagues had the following sobriety test. He considered himself sober if he could write down the first 100 digits of $\pi$. But once he woke up after a night of partying. He did not remember anything what happened last night but there was a piece of paper next to his bed with the first 100 digits of $\pi$ written on it. – Mark Sapir Dec 24 2010 at 22:05
Arthur Benjamin explains some tricks which he uses to compute faster (not as an autistic savant, but something you can learn): ted.com/talks/… – Cristi Stoica Dec 24 2010 at 22:13
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