@grammargirl I knew this already, but I'm not here to mock. You, and the others who are about to (or just did) learn it from your boost, are in today's Ten Thousand:
@grammargirl I used to teach this rule to students when I was helping a teacher by adding enriching activities. I use a visual description where you hold up you ten fingers and count over from the left how many "nines" you need to multiply by. For example, to multiply 1x9 you would drop the leftmost finger and be left with 9 fingers on the right. For 2x9 you would drop the second from the left finger and be left with 1 finger on the left and 8 on the right. They loved it.
@grammargirl I came up with that myself in elementary school and was told it was way more work than just memorizing without thinking about it. Which was why I hated math until I got a Commodore 64 which let me do weird stuff without saying that was a dumb way to do it.
@grammargirl Neat... I know a couple of the ones in thread but if you find it I am looking for the multiplication one where you draw the lines and count the intersections for the numbers like 101x 27 or something...
(I saw it on a vid once but never learned it in school)
@grammargirl I learned this trick decades ago and wish it were taught in schools -- it makes it so much easier! I taught it to my children and grandchildren, too.
@grammargirl I so wish my school had taught us about this! As it was they were in a phase of not really even doing times tables, and they never even mentioned number bonds - my Mam had to show me those, and chant times tables with me on the way to school.
I sometimes think that the whole idea of maths education in the seventies was to make it harder, and remove all aids.
And yet every mathematician I know has a whole array of tricks and shortcuts, and uses them All The Time!
@grammargirl@ElizabethLeeCo cool. I learned to multiply 9 (1-10) on my fingers. Holding up both hands, palms facing you, fold down your right thumb, 9 fingers still up. Fold down only right pointer finger, thumb and eight fingers. Left pinky down (6x9) 5 up, space, 4 up.
Whatever you're multiplying by 9, just multiply by 10 (which is the easiest thing in the world, just add a 0 on the end) and then subtract one of whatever you're multiplying by.
Say 9 x 8
What's 10 x 8? 80.
What's 80-8? 72
9 x 8 = 72
How about 9 x 6 ?
What's 10 x 6? 60
What's 60 - 6? 54
9 x 6 = 54
The easiest number to multiply by is 10, and 9 is just "almost 10".
Also I made a multiplication table practice webpage doodad for mah kiddos that I was proud of, and that they actually tried more than one time apiece xD
@grammargirl I recall my practicing times tables with my dad in 3rd grade — which for me was 1973. He wrote down every single one on its own index card: 5x2=10, 5x3=15, and so on. After dinner he’d help me memorize those index cards. First in order, then mixed up, until I had them down pat.
Today I can recite them perfectly from memory. Is it the optimal way to learn multiplication tables? Probably not, but I sure do know them … and I have a lovely, lasting memory about my father.
Here’s a related trick related to transpositional adding errors.
Suppose you want to add 316+214 and by mistake you add 316+ 124. The 124 is a transpositional error because two digits are swapped. The difference between the correct sum and the sum with the transpositional error will always be evenly divisible by nine. In this example: 530-440=90, 90/9=10.
Try it!
And it is possible to be a word person and a numbers person. 😉
Add comment