It's been about 20 years since I read this, but for people interested in quick mental math, here's a great book on doing fast calculations in your head: Dead Re

It's been about 20 years since I read this, but for people interested in quick mental math, here's a great book on doing fast calculations in your head: Dead Reckoning: Calculating Without Instruments https://www.amazon.com/dp/B00BZE4916/ref=cm_sw_r_tw_apa_i_YuzYEbHK0KR6P https://www.amazon.com/dp/B00BZE4916/ref=cm_sw_r_tw_apa_i_Yu... IIRC it covers everything from multiplication to square roots to logarithms.