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Comment by TrainedMonkey | Hacker News Reader

I honestly have not bothered memorizing times table. I always found it easier to compute correct answer each time I need it. I just memorized few anchors: any number times 10 is easy to compute, from that it is easy to compute any number times 5, so only thing I memorized is number times itself. After that you can turn almost any multiplication into simple subtraction or addition.

For example 7 * 8 = 7*7 + 7 = 49 + 7 = 56

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How on earth did you manage to get out of grade school without having everything up to 12*12 burned into your brain? Where did you go to school?

Oh, how I envy you.

Same situation, still don't know them.. 6*8? Calculator! Now chuck calculus and stats at me, easy. Guess my brain just doesn't like arithmetic.. The start of school was annoying, however late highschool was OK when they stopped asking you infront of the whole class.

Never learned division either.. In any form.

Auckland, New Zealand public schools.

I use the same technique as TrainedMonkey and it has worked really well for me.

I actually have a great dislike for the rote memorization of multiplication tables. During school I had everything up to 12 * 12 burned into my brain, but it was as words rather than numbers and wholly sequential. So, when asked 7 * 8 I would have to run through the whole thing up to that point in my mind - "seven times one is one, seven times two is fourteen, ..., seven times eight is fiftysix" - before I reached the answer and could speak "fiftysix" to show I knew it.

I computed my multiplication tables this exact same way as the great-grandparent during grade and high school. I memorized the tables, but I had this same problem of having to sequentially run through them to compute anything. Eventually in college taking tons of math courses finally ingrained it in my head, but to this day I feel like I must have missed something when everyone else was doing their multiplication tables.

I basically got by without knowing times table and suffering for it until about grade 5 which was when I figured out the trick. After that rest of my grades picked up dramatically as well, as I had figured out that understanding how things work is way better than simple memorization.

So basically laziness carpe diem.

You just need to swap schools when you are beginning times tables. Learn the easy ones in the first school up to say 6, then the next school is already past ten, and assumes you know it. Its easier to work it out than learn it on your own. That's what happened with me.

How in the world do you justify the effort of swapping schools to get out of learning the times tables?

I'm pretty sure he isn't telling you how to get out of learning them, he's answering the OP about how it could have happened that a child wouldn't learn them and would instead have some other system.

Still it's pretty amusing to imagine him as a kid executing an elaborate plan to trick his parents into moving just to avoid learning parts of the multiplication table... ^^;

It sounds like you can swap school to avoid learning anything! Seriously, it sounds like a good way to avoid memorizing historical or political knowledge.

I was never taught politics in school. I learned Scottish history (where I am from) when I was way too young and have forgotten much of it, which is a shame when people ask me about it.

I got to 4th grade before I was finally forced to learn my times tables through 9x9.

Nobody ever taught me the number of days in each month. I was well into my 20s before I had it nailed

I still use my knuckles. Put your fists out in front of you, each knuckle, starting with the left hand, represents a month--the tall ones are long (31 days) the rest are short (28 or 30).

28, 29 or 30. Divide by 4, 100 and 400 to figure out 28 or 29.

I don't understand what you're saying.

I'm a fan of the knuckles mnemonic: Starting on the outside knuckle of either hand, a high knuckle is 31 days, and the valley between knuckles is less than that (generally 30; FU February), then when you get to the other end of the hand, you double-count the knuckle (or count the knuckle on the other hand) for July and August.

30 days hath September, April, June and November

This is my version: "30 days has September, all the rest I can't remember."

That's pretty useless as a mnemonic. "30 days hath September, August, March and December" fits the pattern equally well.

Not if you remember the whole rhyme:

    Thirty days have September,
    April, June, and November.
    All the rest have 31,
    Except February alone,
    And that has 28 days clear,
    And 29 in a leap year.

My substitution works without contradictions there too.

It's 3<cough cough h@ck COUGH>... unless it's February in which case 2@AHEM#

>I honestly have not bothered memorizing times table. I always found it easier to compute correct answer each time I need it

Easier, but slower. It pays off to memorize and know instantly -- and of course you can use the partial multiplications from the times table for larger problems involving calculating the correct answer too.

Slower for what, exactly? How often do you need to do arithmetic - multiplication, in particular - in a hurry with no aids?

Perhaps if you were in a cash business, or a trade involving lots of measurement, but there's not much call for it for a software engineer.

When I need arithmetic, I'm usually sitting down in front of a computer, and almost everything I use can do arithmetic - the bash shell, pry repl, Javascript console, emacs buffer, Google or Wolfram Alpha - there's hardly a piece of software I use that can't do arithmetic.

> How often do you need to do arithmetic - multiplication, in particular - in a hurry with no aids?

Every couple of minutes during my day I do some kind of mental calculation relating to my software development work. Anything from sense checking calculations when debugging, estimating file sizes/memory usage, worrying about when numeric overflow/underflow might be a problem. There are countless situations when being able to do quick calculations comes in handy. Of course I rarely multiply two numbers like 7x8 in isolation any more, but I am immensely grateful how Mrs El Guindi, my fifth grade teacher, drummed into us all fast, accurate mental arithmetic. Thanks to her, I can now look at a spreadsheet of numbers and immediately spot which ones are out of place.

Estimation is subtly different skill from exact arithmetic, IMO.

I regularly do arithmetic when planning meals, playing video games, analyzing my favorite sports team's contract situation, etc. Being able to do it in my head without breaking workflow (to bring up a calculator or even just to type in an equation) is a positive.

>Slower for what, exactly? How often do you need to do arithmetic - multiplication, in particular - in a hurry with no aids?

Almost everyday. Besides being a computer programmer, it's obviously used everytime I buy stuff in multiples (and want to know what I'll pay), everytime I build something (and want to know its dimensions), everytime I cook, etc.

I see memorizing the times table as a form of caching. I use it so frequently and it's so 'cheap' to keep this information in my head that any kind of calculation just isn't worth it.

Plus, it's pretty easy to combine this information to multiply above 12.

Your mention of anchors reminds me of Lucky Numbers:

http://www.ee.ryerson.ca/~elf/abacus/feynman.html

Aka Feynman vs the abacus.

I always just prime factorize and then repeatedly multiply. So 7 * 8 becomes 7 * 2 * 2 * 2 becomes 14 * 2 * 2 becomes 28 * 2 becomes 56.

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