Topic: Multiplication or Division

[ren]

When doing mental math, how do you guys divide? Do you multiply and then guess and check or do you do actual division in your head. Almost everybody I've asked does reverse multiplication but my brother (who has a weirdly mathematical mind) actually divides in his head. However, almost everybody I've asked sucks at mental math so I want to get a broader sample.

I asked him his process and he said that he "root divides". So if I ask him 653/6 he'd go 600/6 = 100, 53/6 = slightly less than 9, final answer: 108 and five-sixths.

Whereas when I get to the 53/6 step, I wouldn't just know that it's slightly less than 9, I'd have to do a bunch of multiplication in my head until I stumble to the write answer.

Oh, I also asked him how he got to the 5/6 part at the end so quickly and he said that at some point he memorized all the results for 1/n up until n=12 which makes doing mental math much, much easier.

[sirean_syan]

I do something like your brother, but I just round to easy numbers near what I want. So, for your 653/6 example I would round to 660/6 and say 110 is a close enough for mental math purposes. Usually, if you're doing mental math exact numbers aren't necessary anyways, so it'd good enough.

[idolminds]

Probably a little of both. I've done the "split" division like your 600/6 example. Breaking it down like that makes it a lot easier. Then whatevers left over I might do some multiplication to find the answer depending on the numbers Im dealing with. Like sy though, I do mental division to get close. If I need an accurate answer I'm likely to bust out a pad of paper and do it long hand.

[ren]

Yeah that's what I do as well. Just round and call it close enough. The conversation started because he can get down to the exact number as fast as most people would round so I wanted to know his process.

I can't imagine finding 600/6 being just as easy to figure out 53/6 but I suppose it's fundamentally the same thing, just with an extra step or two.

Now I wish I had tried harder at fractions when I was 10.

[Quemaqua]

I cry while looking for my calculator.

[Pugnate]

I cry while looking for my calculator.

[ren]

There was this dude at the grocery store that blew my mind the other day. Usually when I go to pay for groceries I know roughly the amount that should ring up, give or take a few bucks. The dude in front of me had calculated it to the exact cent, including a 10% student discount, 15% sales tax and a refund for bringing reusable bags.

[Pugnate]

He was Asian?

[ren]

White, 6'4"ish, athletic build. Pretty dreamy overall.

[Cobra951]

I do tend to split "on the fly" problems into more manageable chunks, from most to least significant.  In division, if the divisor gets too tricky (like 17), I will probably reverse multiply.  If it's too much of a pain, I'll reach for the nearest calculator.  There's even one in my phone.

[W7RE]

I do a combination of reverse multiplication and rounding, but then pay attention to how much I rounded to get an exact answer.

653/6 turns into 660/6

6*11=66 and add the decimal back on and it's 6*110=660

I know I rounded up by 7, so 110 becomes 108 remainder 1.

I hate saying "remainder 1" so I convert the 1/6 into a decimal. I multiple both 1 and 6 by 5 to get 5/30, then realize I can't divide 5 by 3 evenly, now my brain hurts and I'm bored. Oh wait, I CAN divide 5 by 3. 3/5 is 1. As for the 2/3 remaining, 1/3 is .3333~ so it would be .66666~. 1.6666~ is the remainder...

making the final answer 108.166666

I check it with a calculator and find that the final answer should be 108.833333 and now I'm pissed off and confused. I'm now mentally soothing myself with the fact that I'm good at art.





I'm actually really good at math type stuff, I just haven't used any of it in so long that I've forgotten so much, and hate myself for it.

[Quemaqua]

I actually just bought a book on like basic math and pre-algebra stuff. Because I'm so fucking awful, and I'm going to try to go back to school. I've never done any algebra or geometry in my life, and I hate the fact that to educate myself in completely different directions I'm still expected to. I have high respect for math, I won't call it useless, but it's something *I* will never use. Ever. EVER.

EVER.

I hope I can manage to get through it.

[iPPi]

Math is fun.  I was really good at it when I did my undergraduate degree (engineering).  However, that was years ago and I am kind of rusty when it comes to more complicated math (calculus, linear algebra, proofs, etc.).

That said, basic math is like second nature and I can do most mathematical things that one would encounter on a day-to-day basis in my head.

[Quemaqua]

Math is fun the same way that being beaten to death with a rock is fun: eventually it's over.

[ren]

It's amazing how fast the more complicated stuff goes away. I recently had a prof do some fairly basic calculus to derive a formula and it looked like magic. Just a few years in high school I would have been able to do the same thing no problem but now it just seems outside of the realm of possibility.

I actually just bought a book on like basic math and pre-algebra stuff. Because I'm so fucking awful, and I'm going to try to go back to school. I've never done any algebra or geometry in my life, and I hate the fact that to educate myself in completely different directions I'm still expected to. I have high respect for math, I won't call it useless, but it's something *I* will never use. Ever. EVER.

EVER.

I hope I can manage to get through it.

In day to day life the only math that ever really comes up outside of the absolute basics is statistics. It's kind of weird how all of school directs you to learning calculus when stuff like normal distributions, margins of error and probability come up all the time in the news when so many don't understand it.

Pre-algebra just orients you towards more complicated math which can actually come in handy but by itself it's just learning the mechanics. If you have no intention of going further in math you just have to get it over with.

[iPPi]

I considered doing an MBA about two years ago and started studying for the GMAT.  GMAT has high-school level math and geometry.  I was astonished at how bad I was at it.  I was actually struggling to get through it.  In the end though I did not write the GMAT (and did not relearn geometry).

[W7RE]

I got high school credit for my 8th grade algebra class. That left me with 1 more required math credit to graduate. I also needed 2 "academic electives" though, so I just grabbed 4 math credits. Beyond the basic algebra type stuff, I don't remember hardly any of it.

Actually, I remember Pi to 5 decimals, but not because of math class:

[Xessive]

Division I think I actually divide in my head, it makes sense to me. That said I've always sucked at Calculus. To date I have no idea how to factorize, I just use the quadratic equation (or another formula for cubed factors). Geometry was so much easier for me as well.

I found a "math tricks" app on iOS that has all sorts of simple ways of performing calculations. The trick that still amazes me is Chinese multiplication! Someone posted it on OW before.

[Quemaqua]

[13:13] <@idolminds> hey, if you can play D&D you have enough math knowledge to get through the day
[13:13] <Quemaqua> haha
[13:13] <Quemaqua> that should be the math test. can you play a game of D&D against the dean and win?
[13:15] <@idolminds> All the math questions are "Your Charisma is 3, Strength 6, Int 4. You attack an Orc while flat footed. Assumng you are using a battle axe, what is your current modifier?"
[13:15] <Quemaqua> haha
[13:15] <Quemaqua> Math would be so much better like that.
[13:15] <Quemaqua> If a calculating a square root could decapitate a zombie, I'd be all over that shit.
[13:16] <@idolminds> haha
[13:17] <@idolminds> "What percentage of the zombies head has been removed by the shotgun blast?"
[13:17] <Quemaqua> We should write this math book
[13:17] <Quemaqua> If math makes you feel like a zombie, use it to KILL ZOMBIE
[13:18] <@idolminds> that would be pretty awesome. And educational!
[13:18] <Quemaqua> It's like Typing of the Dead but with math
[13:18] <@idolminds> haha
[13:18] <Quemaqua> QUICK SOLVE FOR X! HE'S ALMOST ON YOU!
[13:18] <@idolminds> hahaha
[13:18] <Quemaqua> they'd have to be like really lame zombies who trip over garbage cans and stuff so they take forever to get to you.
[13:19] <@idolminds> Sega really should have stuck with the X of the Dead series
[13:19] <Quemaqua> yeah
[13:19] <@idolminds> Math of the Dead. Physics of the Dead. Gym of the Dead.

[gpw11]

There was this dude at the grocery store that blew my mind the other day. Usually when I go to pay for groceries I know roughly the amount that should ring up, give or take a few bucks. The dude in front of me had calculated it to the exact cent, including a 10% student discount, 15% sales tax and a refund for bringing reusable bags.

I'd be willing to be the dude is buying the same shit every time.  Either that or he's crazy.  Weirdest thing to actually spend time to figure out, even if it is just a few seconds for some people.


Anyway, Cobra's answer is pretty much the same as what I do. I get really fucked with fractions and unit conversions though.