# Math in your head. **URL:** \ **Category:** In My Humble Opinion\ **Created:** [May 21, 2008, 1:18pm UTC](https://boards.straightdope.com/t/math-in-your-head/450093 "2008-05-21T13:18:15Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![beanpod](https://avatars.discourse-cdn.com/v4/letter/b/a9adbd/32.png) [@beanpod](https://boards.straightdope.com/u/beanpod)\ **Post date:** [May 21, 2008, 5:21pm UTC](https://boards.straightdope.com/t/math-in-your-head/450093/21 "2008-05-21T17:21:55Z")
Like **gigi** , but I break it up according to each place and then add the totals together. 300+300 (or 300x2, depending on my mood) = 600 60+60 = 120 5+5 = 10 730 Similar structure for multiplication and adding large numbers. My mom tried to teach me this back in 4th grade or so, but it just didn’t stick. I realized long about 11th grade what she was talking about and went to her excitedly, saying, “Hey, I understand it now!” “What?” she asked. “Addng in my head!” I say. “Well, it took you long enough.” Oh mom, where would I be without your encouragement? (disclaimer: She actually was quite encouraging. She’s just also kinda mean in a lovable mom sort of way.) ---
**Author:** ![panache45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/panache45/32/64_2.png) [@panache45](https://boards.straightdope.com/u/panache45)\ **Post date:** [May 21, 2008, 5:23pm UTC](https://boards.straightdope.com/t/math-in-your-head/450093/22 "2008-05-21T17:23:50Z")
[QUOTE=gigi] If we’re talking addition, put me down for 360+360 = 720 5+5=10 730 [/QUOTE] That’s what I do, but it happens so fast, I just basically look at the numbers and know the answer. ---
**Author:** ![Nancarrow](https://avatars.discourse-cdn.com/v4/letter/n/c37758/32.png) [@Nancarrow](https://boards.straightdope.com/u/Nancarrow)\ **Post date:** [May 21, 2008, 5:27pm UTC](https://boards.straightdope.com/t/math-in-your-head/450093/23 "2008-05-21T17:27:14Z")
[QUOTE=One And Only Wanderers] I do ((2 \* 36) \* 10) + 10, but then, I’m odd [/QUOTE] Not odd at all judging by many of the other posts. Including the one I’d write if it weren’t made redundant by this post. ---
**Author:** ![Nars\_Glinley](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nars_glinley/32/8177_2.png) [@Nars\_Glinley](https://boards.straightdope.com/u/Nars_Glinley)\ **Post date:** [May 21, 2008, 5:46pm UTC](https://boards.straightdope.com/t/math-in-your-head/450093/24 "2008-05-21T17:46:09Z")
3 + 3 = 6 6 + 6 = 12 6(0) + 12 = 72 5 + 5 = 10 72(0) + 10 = 730. Qed ---
**Author:** ![ShelliBean](https://avatars.discourse-cdn.com/v4/letter/s/f9ae1b/32.png) [@ShelliBean](https://boards.straightdope.com/u/ShelliBean)\ **Post date:** [May 21, 2008, 5:51pm UTC](https://boards.straightdope.com/t/math-in-your-head/450093/25 "2008-05-21T17:51:18Z")
[QUOTE=Hampshire] I guess I did it the long way. 5 + 5 = 10 carry the one. \_ \_ 0 6 + 6 + 1 = 13 carry the one. \_ 3 0 3 + 3 + 1 = 7 7 3 0 [/QUOTE] That’s what I do but in my head it looks something like a slot machine, or maybe Connect Four but with numbers instead of little red disks. It didn’t take a terribly long time. Does that make sense? ---
**Author:** ![Shodan](https://avatars.discourse-cdn.com/v4/letter/s/9f8e36/32.png) [@Shodan](https://boards.straightdope.com/u/Shodan)\ **Post date:** [May 21, 2008, 5:51pm UTC](https://boards.straightdope.com/t/math-in-your-head/450093/26 "2008-05-21T17:51:29Z")
I used logarithms, which I find childishly easy. Unfortunately, my answer was 793. Regards, Shodan ---
**Author:** ![Valgard](https://avatars.discourse-cdn.com/v4/letter/v/7feea3/32.png) [@Valgard](https://boards.straightdope.com/u/Valgard)\ **Post date:** [May 21, 2008, 6:14pm UTC](https://boards.straightdope.com/t/math-in-your-head/450093/27 "2008-05-21T18:14:33Z")
If you actually meant multiply, not add, here’s how I did it in my head: 365_365 = (300+65)_(300+65) =300^2 + 65^2 + 2_300_65 300^2 and (2_300_65) are simple mentally. I then had to do 65^2 in my head which you can apply a similar breakdown to: 65_65 = (60+5)_(60+5) =60_60+5_5+2_5_60 =3600+25+600 =4225 And that’s the only hard term in the original question. Now I returned to the original terms which are “simpler” and kept a running total: 300_300 = 90,000 2_300_65 = 300_130 = 39,000 (total 129,000) Finally add in the funny number (that’s how I think of it), 4225 and you get 133,225. Got it right the first time, took a minute or three in my head. I’m pretty good with multiplying two digit numbers mentally. I generally find it easier to express them as (a+b) rather than (a-b) because in the former case, all of your operations are addition whereas in the latter you have some addition and some subtraction, I find that I start making mistakes adding when I should have been subtracting, or vice versa. ---
**Author:** ![El\_DeLuxo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/el_deluxo/32/9043_2.png) [@El\_DeLuxo](https://boards.straightdope.com/u/El_DeLuxo)\ **Post date:** [May 21, 2008, 9:17pm UTC](https://boards.straightdope.com/t/math-in-your-head/450093/28 "2008-05-21T21:17:42Z")
[QUOTE=ParentalAdvisory] How do you personally multiply these two numbers together in your head? You know, the step by step process that you use to come to your conclusion. 365 + 365 [/QUOTE] Whaat? Do you think most people can multiply 3-didjit numbers in their heads? Are folks in general \*expected \*to be able to do that kind of multiplication in their heads? Because there is just no fucking way I can do that, not to save my ass from a firing squad or to win a zillion dollars tax-free. It wouldn’t be a matter of just getting the answer wrong, either; manipulating that many numbers mentally without dicking it out in pencil is flat out impossible for me. I thought it was only real math whizzes who could do shit like that. > [@](#): > > Easy enough I suppose, but I’m no savant. I had to work it out in my head for about 8 seconds. “Easy enough”, s/he says. “8 seconds.” I knew I was just a half-step up from a practical innumerate already (hell, I can’t count past ten if I’ve got shoes on 😉 ) but the inference that intelligent people do sums like that in their heads every day as a matter of course, tells me maybe I oughtta just put on this nice pointy paper hat…and climb up on this five-foot stool back here in the corner…and whimper to myself, very quietly, for the rest of the day . > [@](#): > > What intrigued me about it, was when I later reviewed how I came to the correct answer (yes, for no good reason at all). > > I took 365, and brought it down as far as I could to a 100 base (if you can call it that). So… 300+300 = 600. > > I then took 65 x 2 and thought to myself “what do I have left over after 100?” 15. Okay, so 15x2 is 30. Result 600+100+30 = 730 Now _that_ just literally makes my head hurt trying to follow it. > [@](#): > > how can I improve on getting the answer in a simpler way? The _only_ way I know to find 365 times 365 is with a pencil on scrap paper, and I’d probably need the better part of a minute to dope it out. I’m not trying to mess with you, OP, it just slays me that you, and the folks you aimed your query to, can figure that out in your mind in just seconds, like it was nothing, while I would have to sweat it out like a dog shitting peach pits. ---
**Author:** ![ParentalAdvisory](https://avatars.discourse-cdn.com/v4/letter/p/53a042/32.png) [@ParentalAdvisory](https://boards.straightdope.com/u/ParentalAdvisory)\ **Post date:** [May 21, 2008, 10:14pm UTC](https://boards.straightdope.com/t/math-in-your-head/450093/29 "2008-05-21T22:14:40Z")
[QUOTE=DLuxN8R-13] Whaat? Do you think most people can multiply 3-didjit numbers in their heads? … I’m not trying to mess with you, OP, it just slays me that you, and the folks you aimed your query to, can figure that out in your mind in just seconds, like it was nothing, while I would have to sweat it out like a dog shitting peach pits. [/QUOTE] Would it help if I told you I screwed up my post? I meant Add the two together. As far as _multiplying_ two numbers nearing 100, check this out: [![](https://i.ytimg.com/vi/I9t-gYnPNaw/hqdefault.jpg "Fast Math Trick") ](https://www.youtube.com/watch?v=I9t-gYnPNaw) ---
**Author:** ![Belrix](https://avatars.discourse-cdn.com/v4/letter/b/ed655f/32.png) [@Belrix](https://boards.straightdope.com/u/Belrix)\ **Post date:** [May 21, 2008, 10:40pm UTC](https://boards.straightdope.com/t/math-in-your-head/450093/30 "2008-05-21T22:40:22Z")
I do it in powers of ten: 365+365=(300+300)+(60+60)+(5+5)=600+120+10=730. \ 'cause addition is commutative, right!\ ---
**Author:** ![redtail23](https://avatars.discourse-cdn.com/v4/letter/r/3e96dc/32.png) [@redtail23](https://boards.straightdope.com/u/redtail23)\ **Post date:** [May 21, 2008, 10:51pm UTC](https://boards.straightdope.com/t/math-in-your-head/450093/31 "2008-05-21T22:51:38Z")
300x2 = 600 + 60x2 = 120 = 720 + 5x2 = 10 =730 ---
**Author:** ![eleanorigby](https://avatars.discourse-cdn.com/v4/letter/e/ee7513/32.png) [@eleanorigby](https://boards.straightdope.com/u/eleanorigby)\ **Post date:** [May 21, 2008, 10:53pm UTC](https://boards.straightdope.com/t/math-in-your-head/450093/32 "2008-05-21T22:53:40Z")
Multiply? It says “+”. I know it’s been years since I had any formal math, and almost none of it stuck, but I do believe that is not multiplying you have there. If I was to add those 2 numbers in my head (and that I can do–but no way could I multiply them), I would do 300+300=600. 65+65=130. 600+130=730. Or I might ask one of my offspring. What are children good for, if not for small errands and stray math problems? ---
**Author:** ![rocking\_chair](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rocking_chair/32/2918_2.png) [@rocking\_chair](https://boards.straightdope.com/u/rocking_chair)\ **Post date:** [May 21, 2008, 10:59pm UTC](https://boards.straightdope.com/t/math-in-your-head/450093/33 "2008-05-21T22:59:57Z")
good grief, it gives me a headache just thinking about it. i can not juggle that many numbers in my head and have them land properly. were i to attempt such a thing without a calculator i would…figure on it being about 2 years. if that didn’t fly. it would be 5+5=0 carry a 1… 6+6=12+1=3 carry a 1… 3+3=6+1=7 so that will be seven three zero. if i were multiplying them you may as well sit down for 2 years… here we go: 5 time 5 is 5 carry a 2. 5 times 6 is 0 +2=2 carry a 3, 5 times 3 is 15+3=18 so first line is 1835 onward to the second line… six times 5 is 0 carry a 3, six times six is 6 plus 3=9 carry a 3, six times 3 is 18 +3= 21 so second line is 2190 and still we juggle… 3 time 5 is 5 carry a 1, 3 times 6 is 8 +1=9 carry a one 3 time 3 is 9 plus one is 10 so third line is 1095 at this point my brain shuts down because i am now juggling 3 set of 4 numbers and have forgotten the first set completly and def. need paper or a calculator and start over. i am in utter awe of people who can make numbers dance to their tune. complete and utter awe. ---
**Author:** ![samclem](https://avatars.discourse-cdn.com/v4/letter/s/a9a28c/32.png) [@samclem](https://boards.straightdope.com/u/samclem)\ **Post date:** [May 22, 2008, 12:09am UTC](https://boards.straightdope.com/t/math-in-your-head/450093/34 "2008-05-22T00:09:44Z")
It just magically appears in my head that it’s 730. 🙂 And, my 15 year old daughter’s. Not so, my 17 year old and 21 year old son. If I were _tortured_ to explain in detail, I guess I might say something like 36 +36 is 72, add the half + half, you get 73. But it’s more instantaneous than that. ---
**Author:** ![lobotomyboy63](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lobotomyboy63/32/2900_2.png) [@lobotomyboy63](https://boards.straightdope.com/u/lobotomyboy63)\ **Post date:** [May 22, 2008, 12:42am UTC](https://boards.straightdope.com/t/math-in-your-head/450093/35 "2008-05-22T00:42:15Z")
2=10/5 You could multply by 10 and divide by 5. 365x10=3650/5=730 Or reverse the operations: 365/5=73 x 10=730. Yeah, it’s often harder but sometimes people can get /5 by imagining the number of nickels. ---
**Author:** ![Khampelf](https://avatars.discourse-cdn.com/v4/letter/k/8e7dd6/32.png) [@Khampelf](https://boards.straightdope.com/u/Khampelf)\ **Post date:** [May 22, 2008, 12:44am UTC](https://boards.straightdope.com/t/math-in-your-head/450093/36 "2008-05-22T00:44:05Z")
I use **90wt** ’s method, only I did the 300s first. plus the internal dialogue is like “Two three hundreds is six hundred plus two 65s, 130. Uhh what was the first number, oh yeah 600 plus those two 65s again, 130…730. OK, yeah…” I use ‘round up and subtract the difference’ When the number is close to an ordinal. (That the right term?) ---
**Author:** ![elfkin477](https://avatars.discourse-cdn.com/v4/letter/e/a9a28c/32.png) [@elfkin477](https://boards.straightdope.com/u/elfkin477)\ **Post date:** [May 22, 2008, 2:34am UTC](https://boards.straightdope.com/t/math-in-your-head/450093/37 "2008-05-22T02:34:22Z")
600 + (65x2=130) = 730 [QUOTE=DLuxN8R-13] Now _that_ just literally makes my head hurt trying to follow it. [/QUOTE] That’s what people say when I explain how I do averages in my head. Say you want to average 65, 72, and 85: first, subtract 65 from the higher numbers (so all the numbers are the same, you see) = 7, 20 then add those numbers together = 27 now divide by three (since you now have 3 65s _off over there_)= 9 last add that 9 to 65 = 74 so that’s the average. 74. This works for me with up to 5 or 6 numbers before I get confused by what I’m keeping track of in the second step. ---
**Author:** ![SCSimmons](https://avatars.discourse-cdn.com/v4/letter/s/e495f1/32.png) [@SCSimmons](https://boards.straightdope.com/u/SCSimmons)\ **Post date:** [May 22, 2008, 2:52am UTC](https://boards.straightdope.com/t/math-in-your-head/450093/38 "2008-05-22T02:52:15Z")
Well, first I convert to binary: 101101101 + 101101101. Then, I load the rightmost digit into my A register … Well, the rest should be pretty obvious, right? ---
**Author:** ![ToeJam](https://avatars.discourse-cdn.com/v4/letter/t/dfb087/32.png) [@ToeJam](https://boards.straightdope.com/u/ToeJam)\ **Post date:** [May 22, 2008, 3:06am UTC](https://boards.straightdope.com/t/math-in-your-head/450093/39 "2008-05-22T03:06:57Z")
Doh! Here I was thinking I was quite clever as all I did was: 365\*2 ---
**Author:** ![Northern\_Thalia](https://avatars.discourse-cdn.com/v4/letter/n/ba8739/32.png) [@Northern\_Thalia](https://boards.straightdope.com/u/Northern_Thalia)\ **Post date:** [May 22, 2008, 3:39am UTC](https://boards.straightdope.com/t/math-in-your-head/450093/40 "2008-05-22T03:39:09Z")
As a former roulette dealer, I’ve learned/was taught to add the 100’s first, then 10’s, then 1’s. It is something that you REALLY need to do in order not to lose things in the muck that is your thoughts. And now, I just apply that to everything, which is awesome, because my Arts degree brain is a zillion times faster at basic math than my Science PhD boyfriend’s. 😃 However, you stick some X’s, Y’s, Z’s… or any other symbol, and he wins, hands down! [Previous page](https://boards.straightdope.com/t/math-in-your-head/450093.md?page=1) [Next page](https://boards.straightdope.com/t/math-in-your-head/450093.md?page=3)