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!

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