Imaginary Abacus - Instruction book

Learn to add and subtract in your head by imagining an abacus to calculate the result. Example: Learn to calculate 415-350+233-155+44 quickly, just by imagining an abacus.This book will teach you the skills required to firstly use the actual abacus effectively, then how to use an imaginary abacus (also known as a mental abacus). To follow this book you will need: 1) A Japanese abacus (with at least 13 columns). 2) The accompanying workbook 'Imaginary Abacus - Workbook' (sold separately). This useful and impressive skill would be an asset for anyone.