Tuesday, October 21, 2008

Times table tricks

I'm counting down the days till the exams are over. As you know, these past few days were PSLE marking days, meaning school is closed. Having both kids at home together is no fun. For once, both of them have set aside their differences as they have a common enemy - their mum who has turned into a fire-breathing, revision-driving ogre.

Yesterday, I set some problem sums for Andre. He took the assessment book, then five seconds later, re-appeared at my side and said indignantly, "These are not problem sums!" Huh? "These are CHALLENGING problem sums."

Apparently I should have indicated the difference. Afterall, this is the same boy who, when he telephoned his aunt to tell her about his distinction in his piano exam, said "Auntie Anne, I got a distinction for my piano exam." Pause. "That means I pass, you know."

Back to revision. I know many parents are like me during exam season - we agonise over 1001 details in the school syllabus that our kids have to know and yet somehow elude them. So while I can't help you there, here are a couple of tricks on learning times tables, which hopefully can take away one more little thing your kid needs to know.

My tricks are for the 7, 8 & 9 times tables - the ones that kids tend to struggle with most. (For the others, you just have to do it the old-fashioned way, sorry!)

Many would have heard of this one for the 9 times table, but here it is anyway. Hold up both hands. Eg. if you want to know 9x7, bend the 7th finger (counting from left to right), as shown in pic below. The answer is 6 (all fingers before bent finger) 3 (all fingers after bent finger). 63! Tah dah! Fool proof, this one.


Here's another less well-known one but equally useful trick. It works for multiplication of any two numbers of 7, 8 & 9. Face both palms towards you. The ring finger stands for 7, the middle finger is 8, the index finger is 9. Say you want to know what is 7x8. Make the two corresponding fingers touch, as shown below:

Now count the fingers that are touching and all those below (5), that's your first digit. Then take the number of fingers on one hand above the touching finger (3), multiplied by the number of fingers on the other hand above the touching finger (2), that's your second digit. Answer: 56. Try it!

13 comments:

Anonymous said...

Yes, back then my Indonesian-helper used to talk about the costs of things in her homeland, in terms of tens & hundreds of thousands or millions in that currency. I often wondered if they learn 'mental' or 'finger' math pretty early just from necessity.

YY.

breve1970 said...

Hi Monica

Thanks for sharing again. Method 1 is helpful and creative. Pardon me, I can't quite figure out Method 2, must go through it again. Am quite DUH....

monlim said...

Ann, no worries! Look at the pic again, I'm sure you'll get it. I wish I'd known this as a kid when I was sweating over my tables!

Anonymous said...

My boy's method is a variation of this, where the left and right hand fingers overlap. 7 is represented by the thumb and index, 8 by thumb, index and middle finger, and so on.

So 7 x 8 will give you 3 fingers overlap with 2 fingers - the overlapping fingers give the "ten" digit, in this case "5" while the free fingers, you multiply them, giving 2 x 3 = 6, thus "56". hope i make myself clear.

MD

monlim said...

MD: thanks! sama sama lah... :)

breve1970 said...

Thanks Monica and YY. Will look through again.

Anonymous said...

To calculate 11 x 11, 12 x 16, (two digits), etc, using fingers, watch this video:

http://pslemath.com/main/?cat=17

MD

angiefm said...

I know the 9x one, but the other one? Wah! I totally blur! Maybe because I'm all thumbs! :D Thanks for sharing!

monlim said...

Angie: no lah, maybe it's just my explanation that's not very clear - it's actually very easy if you see it done in person. Andre loves this one, he can never remember his 7s and 8s!

Jo said...

Was browsing thru your maths posts and came across this one - useful 'cause am now trying to teach my dd1 a bit of multiplication. Didn't bother with it at K2 but realised that there you are expected to know some multiplication for P1 ! Oh well - haven't made it to the 6 times table yet so these tricks will come in useful ... once I figure them out myself - Haha!

monlim said...

Jo: Don't worry about multiplication for P1, those are the simple ones like 2s and 3s and I believe they only do those in the 2nd semester. From what I recall, the school does cover it quite thoroughly so the kids shd be fine.

Maths is tricky that way, if the kid is conceptually not mature yet, it can be tough to teach the concept. At k2, Andre just couldn't grasped subtraction, then suddenly in p1, he just got it. So don't worry too much! These tricks are for the higher numbers so you won't have to deal with them till p2 :)

Jo said...

Yeah, I agree about this maturity factor - in fact dd1 couldn't really add in the earlier part of K2 ( 6 + 3 = ???) and then after mid-year she suddenly got the hang of it ! My dd2 is in K2 this year and she's just doing the basics for +/- up to 10 - using her fingers !

I am more concerned about some other P1 topics like the more than / less than & problem sums - my mum was quite stumped this March hols when she tried to sit with dd1 to do some sums ! She complained that a lot of maths questions were not "clearly stated"! Aiyoh...no longer the straightforward +/-, testing comprehension instead !

monlim said...

Jo: Yup, p1 maths can be quite a culture shock. It used to be if the word "more" was in the sum, you'll add, if "less" then subtract. Not anymore! I always felt that p1 maths was more about English and Andre struggled with comprehending the question rather than the sum itself.

But be encouraged that kids are very adaptable - after a year in the system and being exposed constantly to those type of questions, Andre could do them just fine. So I think your dd should be able to cope as well.

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