[Bobalu]

There was method of fast arithmetic developed by a Jewish man while in a concentration camp... gives one amazing skills at fast calculations.

"The Trachtenberg Speed System Of Basic Mathematics".

I have an old copy.. I think it's downloadable for free now.

[roadcat]

"The Trachtenberg Speed System Of Basic Mathematics".

I studied that while in middle school, on my own in the public library. I was a geek and math major. Fast way to do multiplications in your head by criss-crossing columns of digits. My mom still beat me using her abacus.