Try to get hold of this Indian book: Vedic Mathematics or Sixteen Simple Mathematical Formulae from the Vedas (https://www.amazon.com/gp/product/8120801636/ ht

Try to get hold of this Indian book: Vedic Mathematics or Sixteen Simple Mathematical Formulae from the Vedas (https://www.amazon.com/gp/product/8120801636/ https://www.amazon.com/gp/product/8120801636/). I have also heard of a Trachtenberg Speed system (https://www.amazon.com/Trachtenberg-Speed-System-Basic-Mathematics/dp/4871877094 https://www.amazon.com/Trachtenberg-Speed-System-Basic-Mathe...).