My older son is learning his times tables. For me, this brings back painful memories of memorizing columns of data. Fortunately, Kerm has Itinerant Cryptographer for a father. My husband is something of a math junkie. He taught Kerm a little trick for multiplying by nines. It works for 9x1 through 9x10:
* The first digit of the answer is always one less than the number you are multiplying by,
and
* the digits of the answer have to add up to nine.
So, let's say that you are multiplying 9 x 3. The first digit of the answer starts with a "2". And 9 minus 2 is 7, so the answer is 27.
9 x 1? The first digit would be zero. Nine minus zero equals nine, so the answer is 9.
Isn't that neat? (Where was Itinerant Cryptographer when *I* was learning my times tables?
Photo credit: Paul Joseph, through a Creative Commons Attribution 2.0 License via Flickr.
2 comments:
My mother taught me the same trick when I was learning this table. It is one of the only Times Tables that I still remember easily to this day.
It's embarrassing to admit this, but when I was quizzing Kerm about the 9's time tables, I realized that I had forgotten 9 x 12. I had to stop and think about it before I could answer. I guess it hasn't come in as handy as the others!
Thanks for stopping by, emilieDwolf!
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