How well do you know the multiplication table?

I very much value the ability to do simple math quickly in my head. I can't imaging having gone through the computer-based and technology-based career that I've had without having it.

 

When I was in elementary school, we drilled and drilled the multiplication tables for hours. (We only went up to 12. I wish we'd gone up to 15 like some schools.)

So, for years it was automatic. Then calculators became cheap and ubiquitous and I found myself pulling one out automatically when I had any math to do whatever. The result was a terrible erosion of my ability to do any math in my head.

 

odd question lol. i remember that 42 is 6*7 because thats the meaning the life, the universe, and everything (hitchhikers guide to the universe)
i still struggle with 6*8 and 7*8, sometimes its pretty frustrating.

 

The ability itself is what I have found extremely useful, and not just limited to my professional life. For example, I often find that statistics given even in reputable publications like Time Magazine (just as an off-the-cuff example, they are not, by far the worse) are obviously wrong and don't pass even the most simple smell test. I can SEE very quickly that they HAVE to be off because simple in-my-head rough calculations that give me estimates for what they would imply for the underlying facts show clearly that they just can't be right.

I'm old enough that when I was in school we drilled arithmetic seriously including memorizing the squares up to 15 (and when I got into computers, 16 became ingrained).

 

I have multiplication tables memorized up to 9x9=81. 10 is super easy so there's no need to memorize up to 10x10, and 11x11 is also quite easy and no need to memorize. 12 I know somewhat well due to there being 24 hours in a day, but something like 12x7 or 12x11 would be problematic for me.

 

I've pretty much got 12*10 nailed down.

 

That's interesting. These are things I don't typically think about. I always just assumed every (especially physics undergrad) body had essentially the same single digit multiplication table memorized. It's just automatic for me, but maybe if it hadn't been hammered into my brain in grade school I wouldn't have the same "knee-jerk" capacity I have now. I don't know. Maybe there's a window for learning it so you can bypass the calculation process. After that, you have to do things like add 12, etc., although my guess would be that with focused training, you should be able to memorize these, and even 2 digit numbers up to say, 20 or more. Whatever the case, at the end of the day it's simply a time saving tool, that's all, kind of like memorizing the power rule in calculus.

 

It's fine to do computation whatever way you'd want to. I do all 2 digit multiplications mentally and am always slower than others too. Why do you want to do this faster than others ?

 

I always do 7*3 = 21, then *2 = 42 for 7*6. I hate that one. And 7*8 .

 

How about Roman numeral multiplication : (XVI ) x (ML) =? Or base 16 : (AB ) (3C)=?
For the first : Multiply L by I , carry a V..... So quit complaining, you have it much easier
than Roman kids did ( and maybe Klingons or something use base 16 ).

 

Alright, fine, fine, I'll get off your lawn, sorry dude.

 

I can do up to 20x20 or a little more. In India, in 5th grade (in CBSE), they have to know up to 20x20.

 

There is a nice trick for squaring:

## a^2 = a^2-b^2+b^2 = (a-b)(a+b) ## , then, given an a, you choose the right b; say

a=28 . Then you can choose b=2 , and get : ## 28^2 = (28+2)(28-2)+ 2^2 = 30(26)+4 ## . Usually

choose a to get a number ending in 0 ; you just need to be able to compute 26(3), which is not easy, and then just put a zero on the right, but not impossible. If you know the squares up to 20 , you can figure something like:


## 987^2 = (987+13)(987-13)+13^2 = 974(1000)+13^2 ##

And you can always reduce multiplication to a squaring issue when the two numbers being multiplied have different parity.

 

And how about the trick of dividing by 2 and appending a zero when multiplying by 5 ? This is just because 5 =10/2. Say you want to find 36 x 5 . Then 36/2 =18 and 18 x 10 =180. By the same trick, dividing by 5 is multiplying by 2 and appending a zero :

240/5 240 x2 =480 , 480/10= 480.

 

It seems many people are weak for 7*8 (including myself ).

Are there any elementary school teachers here (or lurking) who can explain why this is? Are we teaching multiplication tables wrong? Or is this a consequence of having a base-10 number system?

 

Yes, it's probably a consequence of base-10 in addition to the fact that we only go up to 10 or 12 in the US (or did when I was a kid). The base-10 leave 7 and 8 the numbers with least obvious mnemonics to break it down quickly (in terms of multiplication). For instance, the sums of digits and how they relate to their multiples is seen here:

http://www.sjsu.edu/faculty/watkins/Digitsum0.htm

This accounts for why 3, 6, and 9 are easy. 2 and 4 are easy because it is just doubling twice (since we're staying on 10-12, all fairly easy to double twice, only leading up to 48 max. 5 and 10 are easy because their last digit is always the same as well as the division by two and multiplication by 10 above.

With 8, you have to double three times now, and that gets more tedious, but you can handle it for the most part on the smaller numbers, especially because you can use the mnemonics of the other number for 8x[1-6]. For whatever reason, we learn the diagonal well so we know 8x8, and 9 and 10 we're back to some of the simplest mnemonics. But that 7 is fairly large prime with no digit addition mnemonics, a sum of digits pattern that aren't easy to remember, and a pattern for last digit that's not easy to remember. So when 7 and 8 clash, our lowest hanging fruit is to to double 7 3 times or go 7*4*2 = 28*2, and we don't go up to 28 with the multiplication table, so we haven't memorize 28*2 and so we have to do the computation in our head the one time we need it every blue moon.

But that's just my personal theory :uhh:

 

I'm weak in remembering my past, so I don't know how I did it in elementary school. But I think practice, and not repeating it over and over and over, stuck into my mind, multiplication table till 9x9. For other more complicated multiplications, I have tricks.
For example for 34 by 62. It is (30+4)x62=3x10x62+4x62.
And that's easy to calculate in mind. But further simplification is still possible: 3x10x62+4x62=60+3x10x60+4x60+8.
This way, I can even multiply two 3digit numbers in mind. But for more digits, this becomes messy and actually you don't have to do that much calculation in your mind.
Also division can be simplified. If you want to divide a number with a zero at the end by 2(5), just throw out the zero and multiply the remaining by 5(2). You also can factorize the divisor(not to primes) and divide the dividend by one factor and divide the result by the next factor and so on.
I think I'm good at such calculations by mind. Sometimes I even differentiate or integrate in mind and not just for simple functions. I only need to be able to focus enough.

 

Heh, I'm actually in the exact same boat as you, OP. Sophmore, undergrad, 20, taking calculus 2 and I don't have my memorization tables down.

Funny thing. I've always had a really hard time with rote memorization things. It's why I hate these calculus sections with trigonometry. Thank goodness for flash cards!

It's really funny, actually, I'm so huge into the sciences now and I profess to love mathematics, yet I'm not particularly advanced in them and I really wasn't the brightest kid in Elementary...