# Why do I hate 8?June 5, 2007 10:51 AM   Subscribe

Why is it hard for me to perform simple mental calculations with certain numbers?

I've always had a problem performing mental multiplication with the numbers 8 and 6, to the point that I have a visceral reaction to those numbers - I just don't like them. I'm not talking complicated calculations here, just your basic simple single-digit, multiplication table stuff (i.e. "8 times 7" or "6 times 4"). I like math and never had any problems with college-level algebra, calculus, etc., so I can't figure out why it takes me substantially more time to perform exceedingly simple mental calculations involving 6 and 8.

Is this a common thing...do most people have inexplicable difficulty using certain numbers? Is there a name for this type of minor mental "blip" or deficiency?
posted by lalex to Science & Nature (19 answers total) 3 users marked this as a favorite

I'll be honest, 6x8 and 6x7 ALWAYS take me longer than any other times-table mental math.... I think part of it is because, of the first 10 numbers, they're the highest you get without having a shortcut (the put-down-one-finger rule for x9, and x10... well that's just easy).

"I just don't like them." I'd say that part is probably psychological, not mathematical.
posted by Zephyrial at 10:59 AM on June 5, 2007

Strange as what your saying seems, I understand. When I multiply by 6, I usually triple the number and then double it. For 8, perhaps just double it three times?

I'd think 7 is trickier since it's prime.
posted by JMOZ at 11:05 AM on June 5, 2007

I was thinking of asking this exact same question. I'm no slouch at math but the number 8 in particular always gives me problems when I do mental calculations. I hate it because when I'm trying to do long multiplications in my head there's an extra pause.

I'm a fairly visual person mathematically and I've gone so far as to sketch out a visual representation of the numbers in my head. I always attributed the problem to discontinuities in my visual representation but perhaps I'm overthinking it...
posted by vacapinta at 11:10 AM on June 5, 2007

I always had trouble with the 8s in the times table. Maybe I was just tired of the subject by the time I got to them. 6*8 and 7*8 always seemed the hardest to remember somehow (unlike the 9s, which were fun).
posted by DarkForest at 11:12 AM on June 5, 2007

Weird. Me too. I've never had trouble with the 8s times tables - but I just memorized those, long ago. Simple addition, though, with 8s, makes me stumble and I have to use my fingers. Or, instead of adding 8+7, I'll add 9+7 and subtract 1.

I hope someone comes along who can actually explain why 8s give us trouble.
posted by rtha at 11:25 AM on June 5, 2007

6*8, 7*8, and 6*7 were toughest for me too. In general, sevens piss me off because there's just no tricks for sevens at all. What kind of number refuses to give up a single freaking divisibility rule? Yeesh. Especially coming after the fives, which are so well behaved, the six and seven times tables are a horror.

I have a lot of numbers I don't like, though. Give me a list of integers and I'll generally pick out about a quarter of them that I react badly to.
posted by crinklebat at 11:27 AM on June 5, 2007

I always had problems with multiplying 6, 7 and 8 too. It actually held me back in my early years at school a bit, until the time we were allowed to use calculators.
(Not really an answer, but just adding to the 'it's not just you' evidence)
posted by liquidindian at 11:31 AM on June 5, 2007

I've always had trouble with 4s and 9s. For some reason I just cannot get their multiples stuck in my head. Like you I've developed an emotion of some kind around them. If I'm doing a calculation and there's a chance of a 4 in it I just get upset. Not only because I can't do it either. I too just don't like it.

However, I suck at math and am doing upgrading to get into college so that may be a reason for my anger.

Be neat if there was an official word for this.
posted by beautifulcheese at 11:31 AM on June 5, 2007

I have never minded integers, but calculations with e, the base of the natural logarithms have always been hard and unpleasant. I'm OK with other transcendent numbers, it's just that one that bugs me. I guess it's just an irrational thing.
posted by RussHy at 12:05 PM on June 5, 2007 [2 favorites]

I guess it's just an irrational thing. Oh, I think it's more complex than that.
posted by Neiltupper at 12:14 PM on June 5, 2007 [2 favorites]

oh man... i can't tell you, personally, why these numbers frustrate you so much, but i can certainly sympathise. i have been dealing with fear of multiplying (or adding, subtracting or dividing, as well, frankly) by eight for thirty of my 38 years. christ how i hated times tables! it's refreshing to hear that other people have stumbling blocks with these, because all this time i thought it was just me.

it probably didn't help me that i went to an old-fashioned rural grade school and had a sadistic third grade teacher who felt beating answers out of us was the solution. i mean, i'm sure fear of getting the ruler/cane/paddle was highly motivating to most of my classmates, but not for me - it just served to make my mind go blank.

i gave up on math at that point, and i've been a math illiterate ever since. it was definitely the 6,7,8 times tables that first gave me fits tho, with 8 being the most difficult - there just don't seem to be any handy tricks for memorising them.

if i had a buck, tho, for every time i got caned or stood in a corner for blanking out, i'd have been the wealthiest 8 year old in ohio.
posted by lonefrontranger at 1:16 PM on June 5, 2007

No guess to why, but here's another person with a 6-7-8 problem. Can't add them to each other, can't multiply them either (except I know their squares). Makes playing cribbage against me that much more fun.
posted by TG_Plackenfatz at 2:40 PM on June 5, 2007

There's a trick for remembering even digits multiplied by 6: whatever the multiplier, the total will have 2 digits: the first digit will be half the multiplier, and the second digit will be the multiplier itself.

Which is all an elabortate way of saying: half of 2 is 1; therefore 6 x 2 = 12.
2 is half of 4, so 6 x 4 = 24
3 is half of 6, so 6 x 6 = 36
4 is half of 8, so 6 x 8 = 48

The trick for multiplying single digits by 9 is to remember that the 2 digits of the final result will always add up to 9; also, that the first digit of the result will be one less than the multiplier.

In other words: 2 - 1 = 1; so 2 x 9 = 18 (because 1 is 1 less than 2, and 1 + 8 = 9)
3 - 1 = 2; so 3 x 9 = 27 (because 2 is 1 less than 3, so 2 is the first digit, and 2 + 7 = 9, so 7 is the second digit)
4 - 1 = 3; 4 x 9 = 36
5 - 1 = 4; 5 x 9 = 45
6 - 1 = 5; 6 x 9 = 54
...etc.
posted by scody at 3:00 PM on June 5, 2007 [2 favorites]

I can't do sixes, sevens or eights either. Addition or multiplication both freeze my brain when these digits creep in. To the extent that I effectively can't handle numbers. Theory and concepts are fine, however.
posted by blue_wardrobe at 6:18 PM on June 5, 2007

crinklebat: In general, sevens piss me off because there's just no tricks for sevens at all. What kind of number refuses to give up a single freaking divisibility rule?

Here's one that may be useful at some time: remove the rightmost digit, multiply it by two, subtract it from the rest of the number. The result should be divisible by 7.
28 -- 2 - (8 * 2) = -14, divisible by 7.
63 -- 6 - (3 * 2) = 0, divisible by 7.
329 -- 32 - (9 * 2) = 14, divisible by 7.
2177 -- 217 - (7 * 2) = 203, who knows? But:
203 -- 20 - (3 * 2) = 14, divisible by 7.

Not as easy to do in your head as, say, the rule for 3's, but perhaps easier than mental division.
posted by lostburner at 6:21 PM on June 5, 2007

You can tackle seven the same way that you tackle nine:
7 * 6 = (10 - 3) * 6 = 60 - 18 = 42

Since seven is prime and does not occur in the base of our number system, it makes it the most difficult to compute with directly.

Also consider: 7 * 6 = (6 + 1) * 6 = 6 * 6 + 6 = 36 + 6 =42

If you can square numbers quickly (or memorize a bunch of common squares) you can basically multiply any two-digit numbers very quickly using the identity:

(x - y)(x + y) = x^2 - y^2 and

Examples:
38*42 = 40^2 - 2^2 = 1600 - 4 = 1596
51*69 = 60^2 - 9^2 = 3600 - 81 = 3519

If the numbers don't differ by an even number just combine with the second 6*7 trick above:

16*11 = 11+15*11 = 11+13^2-2^2 = 11 + 169 - 4 = 176

(Of course, there's a good trick for multiplying by 11 in general...)
posted by mharper3 at 7:49 PM on June 5, 2007

Just to provide an alternate data point, I never found calculations with 6 or 8 harder than calculations with any other numbers.
posted by number9dream at 7:59 PM on June 5, 2007

Mirror images are very common mistakes in the first writings of children, and 0 and 8 are the only digits which are their own mirror images.

I composed a long, labyrinthine answer to this question yesterday, in which I tried to argue that asymmetrical letters and digits require the development of different handling in the brain to suppress mirror writing, but that symmetrical symbols can hang around in the 'original' mode, and that this could make them harder to access visually in mental arithmetic. Thank goodness for whatever shred of sense kept me from posting that, but I do think an interesting answer could lie in that direction.

8 is no different for me than the other digits, but my acquaintance with numbers was somewhat belated; I was deeply humiliated at the end of second grade when it was dramatically revealed to the teacher and the entire class that I could not count to twenty. Numbers remained without form and void to me me until trig class in high school, when I suddenly started to see the sequence of natural numbers as a collection of sine waves, with a new, progressively lower, frequency beginning at every prime, so that big multi-prime composites were vast brier patches of waves crossing the line, and any power of a single prime was a relatively bare spot.
posted by jamjam at 11:14 AM on June 6, 2007

For seven you should really just Learn It.

7*5 = 35
7*6 = 42
7*7 = 49
7*8 = 56
7*9 = 63

It's way less work.
posted by floam at 5:58 PM on August 21, 2007

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