Figuring: The Joy of Numbers – Shakuntala Devi

As a child I was fascinated by mathematics, but especially by tricks and shortcuts that could be done. I started reading Martin Gardner’s section of Scientific American when I was eleven or twelve years old, I don’t claim to have understood all of it but each month my knowledge of recreational mathematics was stretched just that little bit more. I’ll cover one or more of his books in a later blog. However in 1977, when I was fifteen, this book was published and it was written by somebody who, at least partly, earned her living from amazing feats of mental arithmetic, I had to get a copy, and this book is still on my shelves today. Some of it I already knew but there were whole sections where she explained how to do tricks that I had seen done but which had baffled me such as calculating the day of the week for any date given to you or working out square and cube roots in your head. I remember practising these tricks for hours until I could do them too.

The book starts of simply by looking at each of the digits 0 to 9 in detail, explaining what is special about each of them and giving tips around multiplying and dividing by them, patterns in their multiplication tables etc. She then moves on to chapters about multiplication, addition, division and a very short chapter on subtraction. These chapters not only suggest shortcuts, which I still use today, to perform such calculations but ways to quickly check if the answer you get makes sense such as casting out nines. The book really caught my attention however when we reach calculating squares, cubes, square roots and cube roots. Amazingly cube roots which non mathematicians would assume to be much more difficult then square roots are actually very simple and fifth roots are even easier, square roots proved to be quite tricky. But just to see how easy extracting a cube root lets look at all you need to know, worryingly forty five years later I can still remember this:

  • 1 cubed = 1
  • 2 cubed = 8
  • 3 cubed = 27
  • 4 cubed = 64
  • 5 cubed = 125
  • 6 cubed = 216
  • 7 cubed = 343
  • 8 cubed = 512
  • 9 cubed = 729

Assuming that we are starting with 474,552 (which is 78 x 78 x 78) how do you get the right answer? Well first of all look at the thousands i.e. 474, this comes between 343 and 516 so the first digit is the cube root of the lower number which is 7. Next you will notice that all the cubes in the list above end with a different number and you just need to find the one that ends with the same digit as the number you are trying to extract the root of which in this case is 2 which matches 512 or 8 cubed and there we have the answer, the 7 from the thousands value along with the 8 from the final digit gives the required answer of 78. Notice that it was simply a case of knowing the first nine cubes and no actual calculation was performed on 474,552 in order to get the right answer.

Calculating the day of the week is a bit more tricky as you need to memorise four tables, admittedly the first of which is simply the first four values from the seven times table so this barely counts as a table and the working out is also more involved. I can’t do this in my head anymore and frankly with the all pervading computers or mobile phones with calendars on them what was once a occasionally handy ability is now of no use whatsoever as you are rarely that far from a device where you can look up the day for a specific date if you need it. When I was a teenager however this was quite impressive at least amongst the other maths fans at school and I got to be pretty quick at it.

The book finishes with chapters on special numbers and finally tricks and puzzles most of which, even then, I had already encountered but this book stretched still further my mathematical skills and I loved it. It has been great fun reading it again and finding out what I remembered and what I had forgotten. Shakuntala Devi died at the age of 83 in 2013 and wrote several books on mathematics along with astrology and oddly ‘The World of Homosexuals’ which she claimed was inspired by her marriage to a homosexual man but Figuring: The Joy of Numbers is probably her best known work, at least outside India although sadly it appears to now be out of print. If you know a child interested in mathematics I suggest trying to get a copy for them, it really is a joy.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s