I can now multiply quickly by 99

Book cover for Rapid Math Tricks and TipsLast night I was making some space on my shelves for a couple of books on PHP and MySQL that I’d got back from my friend Mike when I came across this book: Rapid Math Tricks and Tips: 30 Days to Number Power by Edward H. Julius (Wiley, 1992) ISBN 0-471-57563-1.

I’ve always loved maths; maybe something to do with Dad having been a draughtsman and electrical engineer. At high school the only Certificate of Sixth Year Studies (CSYS) I took was in Maths, Paper II: Calculus. I really wish that I’d had this book while I was at school. It’s brilliant!

The book is a 30-day self-teaching guide designed to help you master common maths problems without a calculator. It uses tips and techniques to teach the basics of multiplication, division, subtraction and addition as well as estimation and more advanced techniques.

Last night I learned in less than five minutes how to “Rapidly multiply any one- or two-digit number by 99 (or 0.99, 0.0, 990, etc.)”. Here’s how:

  1. Subtract 1 from the number to obtain the left-hand portion of the answer
  2. Then subtract the number from 100 to obtain the right-hand portion

For example, 15 x 99:

  1. Subtract: 15 – 1 = 14 (left-hand portion of the answer)
  2. Subtract: 100 – 15 = 85 (right-hand portion of the answer)
  3. Combine: 1,485 is the answer

Another example, 430 x 0.99:

  1. Disregard the zero and decimal point, and subtract: 43 – 1 – 42 (left-hand portion of the answer)
  2. Subtract: 100 – 43 = 57 (right-hand portion of the answer)
  3. Combine: 4,257 (intermediary product)
  4. Apply Test of Reasonableness: 0.99 is almost 1, so 430 x 0.99 must equal just under 430. Insert a decimal point within the intermediary product to obtain the answer, 425.7.

How cool is that!

Published by

Gareth Saunders

I’m Gareth J M Saunders, 47 years old, 6′ 4″, father of 3 boys (including twins). Scrum master at Vision Ltd, Dundee. Latterly, web architect and agile project manager at the University of St Andrews and former warden at Agnes Blackadder Hall. Enneagram type FOUR and introvert, I am a non-stipendiary priest in the Scottish Episcopal Church, I sing with the NYCGB alumni choir, play guitar, write, draw and laugh… a lot.

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