# 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:** 18\ **Page:** 3
**Author:** ![Amblydoper](https://avatars.discourse-cdn.com/v4/letter/a/fbc32d/32.png) [@Amblydoper](https://boards.straightdope.com/u/Amblydoper)\ **Post date:** [May 22, 2008, 4:05am UTC](https://boards.straightdope.com/t/math-in-your-head/450093/41 "2008-05-22T04:05:01Z")
[QUOTE=Northern Thalia] …my Arts degree brain is a zillion times faster at basic math than my Science PhD boyfriend’s. 😃 [/QUOTE] That is because he is integrating the function y=365, and evaluating the result at 2. 😉 ---
**Author:** ![CanvasShoes](https://avatars.discourse-cdn.com/v4/letter/c/74df32/32.png) [@CanvasShoes](https://boards.straightdope.com/u/CanvasShoes)\ **Post date:** [May 22, 2008, 4:19am UTC](https://boards.straightdope.com/t/math-in-your-head/450093/42 "2008-05-22T04:19:28Z")
I multiply the 2 300s, then the 2 65s. If it’s an odd number like 367 and 379, I’d need to break it down further. I’d separate out the 600 (2\*300), Then add the 6 and 7 together (13) and set that aside in my head and then finally the 9 and 7 (16). then to add the 13 and 16, either 29 will just pop into my head, or I may need to break it down further depending upon the number, 10+10= 20+6 and 3 is 9, all in is 29. (right? I could still be wrong, this is why I have to “break it down” at all, I’m not that great at math in my head). Bottom line, for odd or unusual numbers I usually have to break them apart into numbers ending in 0 or 5, or down to just single digits, and than add the parts back together. This is why God gave us the great equalizers, calculators and Excel!! 😃 ---
**Author:** ![Boozahol\_Squid\_P.I](https://avatars.discourse-cdn.com/v4/letter/b/d9b06d/32.png) [@Boozahol\_Squid\_P.I](https://boards.straightdope.com/u/Boozahol_Squid_P.I)\ **Post date:** [May 22, 2008, 5:15am UTC](https://boards.straightdope.com/t/math-in-your-head/450093/43 "2008-05-22T05:15:18Z")
[QUOTE=KRM] 3 + 3 = 6 6 + 6 = 12 6(0) + 12 = 72 5 + 5 = 10 72(0) + 10 = 730. Qed [/QUOTE] Me too ---
**Author:** ![lizardling](https://avatars.discourse-cdn.com/v4/letter/l/f4b2a3/32.png) [@lizardling](https://boards.straightdope.com/u/lizardling)\ **Post date:** [May 22, 2008, 7:11am UTC](https://boards.straightdope.com/t/math-in-your-head/450093/44 "2008-05-22T07:11:54Z")
[QUOTE=ShelliBean] 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? [/QUOTE] Me too. It’s a visual combination of numbered gear wheels and scratching out and carrying the ones on that cheap blue lined paper from elementary school. I’m going to have to remember the breaking down trick. (300 + 300, 60 + 60, 5+5) ---
**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 22, 2008, 7:31am UTC](https://boards.straightdope.com/t/math-in-your-head/450093/45 "2008-05-22T07:31:35Z")
[QUOTE=ParentalAdvisory] Would it help if I told you I screwed up my post? I meant Add the two together. [/QUOTE] . Oh! _That_I can do! I’d bust it down to 350+350 is 700 and 15+15 is 30 and so it adds up to 730. I didn’t think I could do it the proper way like we were taught in gradeschool in my head, where you havet to carry the numbers over to the next column. if i can have a couple of minutes, rhree alone with a scratch pad, pencil and at least one cigarette. > [@](#): > > As far as _multiplying_ two numbers nearing 100, check this out: > > [http://youtube.com/watch?v=I9t-gYnPNaw](http://youtube.com/watch?v=I9t-gYnPNaw) uuuhhh ooooh…am i in n-nu-numbermovie trouble now? i’m sorry pleeeze! Help@!Help ---
**Author:** ![Indian](https://avatars.discourse-cdn.com/v4/letter/i/c0e974/32.png) [@Indian](https://boards.straightdope.com/u/Indian)\ **Post date:** [May 22, 2008, 7:47am UTC](https://boards.straightdope.com/t/math-in-your-head/450093/46 "2008-05-22T07:47:34Z")
\*\*My way \*\* 360 x 2= 720 5x2=10 adding both = 730 … pretty quick just some seconds… ---
**Author:** ![Nava](https://avatars.discourse-cdn.com/v4/letter/n/da6949/32.png) [@Nava](https://boards.straightdope.com/u/Nava)\ **Post date:** [May 22, 2008, 8:21am UTC](https://boards.straightdope.com/t/math-in-your-head/450093/47 "2008-05-22T08:21:33Z")
Those two numbers are easy, I don’t need to add them. I’ve seen so many “years” tables it’s just in automatic, see. Now, if you’re talking 366+366, then it’s “365+1+365+1, so 730+2, so 732” ---
**Author:** ![LouisB](https://avatars.discourse-cdn.com/v4/letter/l/dfb087/32.png) [@LouisB](https://boards.straightdope.com/u/LouisB)\ **Post date:** [May 22, 2008, 10:25am UTC](https://boards.straightdope.com/t/math-in-your-head/450093/48 "2008-05-22T10:25:54Z")
[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 Easy enough I suppose, but I’m no savant. I had to work it out in my head for about 8 seconds. 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 Does anybody else do this, and how can I improve on getting the answer in a simpler way? [/QUOTE] IIRC, this is the method we were taught in a distributive education class my senior year in high school. (1958) ---
**Author:** ![Colophon](https://avatars.discourse-cdn.com/v4/letter/c/f05b48/32.png) [@Colophon](https://boards.straightdope.com/u/Colophon)\ **Post date:** [May 22, 2008, 11:12am UTC](https://boards.straightdope.com/t/math-in-your-head/450093/49 "2008-05-22T11:12:37Z")
[QUOTE=Mangetout] Twice 36 is 72, so twice 360 is 720 5+5 is 10 720+10 = 730 [/QUOTE] Like this, pretty much. I say “36 and 36 is 72, so it’s 720 plus the two fives which makes 730.” If you wanted me to _multiply_ 365 x 365 in my head, I’d say 365 x 3 = 730 + 365 = 1095 gives me 109500 365 x 6 = double the above, so 2190 gives me 21900 365 x 5 would be 1095 + 730 = 1825. Add them up and you have 131400 + 1825 = 133225. I hope that’s right. ---
**Author:** ![Colophon](https://avatars.discourse-cdn.com/v4/letter/c/f05b48/32.png) [@Colophon](https://boards.straightdope.com/u/Colophon)\ **Post date:** [May 22, 2008, 11:19am UTC](https://boards.straightdope.com/t/math-in-your-head/450093/50 "2008-05-22T11:19:52Z")
The part I find tricky with multiplying three figure numbers like the above, though, is remembering the intermediate results while calculating the next bit. ---
**Author:** ![Amblydoper](https://avatars.discourse-cdn.com/v4/letter/a/fbc32d/32.png) [@Amblydoper](https://boards.straightdope.com/u/Amblydoper)\ **Post date:** [May 22, 2008, 11:57am UTC](https://boards.straightdope.com/t/math-in-your-head/450093/51 "2008-05-22T11:57:11Z")
[QUOTE=Colophon] If you wanted me to _multiply_ 365 x 365 in my head, I’d say … Add them up and you have 131400 + 1825 = 133225. I hope that’s right. [/QUOTE] You are right, and that is a clever system, better then mine. The only problem I see is if one of the digits is 7, although I have a fear of the number 7, so I might be biased. ---
**Author:** ![Engineer\_Dude](https://avatars.discourse-cdn.com/v4/letter/e/9fc29f/32.png) [@Engineer\_Dude](https://boards.straightdope.com/u/Engineer_Dude)\ **Post date:** [May 22, 2008, 12:27pm UTC](https://boards.straightdope.com/t/math-in-your-head/450093/52 "2008-05-22T12:27:00Z")
In my head, I said 350 = 3.5 hundred, so: (350+350) + (15+15) = 700 + 30 = 730 ---
**Author:** ![Valgard](https://avatars.discourse-cdn.com/v4/letter/v/7feea3/32.png) [@Valgard](https://boards.straightdope.com/u/Valgard)\ **Post date:** [May 22, 2008, 4:15pm UTC](https://boards.straightdope.com/t/math-in-your-head/450093/53 "2008-05-22T16:15:08Z")
[QUOTE=Colophon] The part I find tricky with multiplying three figure numbers like the above, though, is remembering the intermediate results while calculating the next bit. [/QUOTE] That’s why I try and break it down a little differently so that I’ve got some very simple terms that I can handle at the end and the more complex bits get handled in one slug. Then I can just add the simple parts (in my example, stuff like 300\*300) to finish. ---
**Author:** ![Leaffan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leaffan/32/299_2.png) [@Leaffan](https://boards.straightdope.com/u/Leaffan)\ **Post date:** [May 22, 2008, 4:48pm UTC](https://boards.straightdope.com/t/math-in-your-head/450093/54 "2008-05-22T16:48:14Z")
I just looked at it and thought “730.” I didn’t need to do anything. ---
**Author:** ![Rebo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rebo/32/13005_2.png) [@Rebo](https://boards.straightdope.com/u/Rebo)\ **Post date:** [May 22, 2008, 4:52pm UTC](https://boards.straightdope.com/t/math-in-your-head/450093/55 "2008-05-22T16:52:47Z")
I rounded up to 400, x 2 = 800. Then 35 x 2 = 70; 800 - 70 = 730. ---
**Author:** ![ParentalAdvisory](https://avatars.discourse-cdn.com/v4/letter/p/53a042/32.png) [@ParentalAdvisory](https://boards.straightdope.com/u/ParentalAdvisory)\ **Post date:** [May 23, 2008, 6:39am UTC](https://boards.straightdope.com/t/math-in-your-head/450093/56 "2008-05-23T06:39:38Z")
[QUOTE=Leaffan] I just looked at it and thought “730.” I didn’t need to do anything. [/QUOTE] Did you learn how to do this, or is it just natural for you? ---
**Author:** ![Valgard](https://avatars.discourse-cdn.com/v4/letter/v/7feea3/32.png) [@Valgard](https://boards.straightdope.com/u/Valgard)\ **Post date:** [May 23, 2008, 5:24pm UTC](https://boards.straightdope.com/t/math-in-your-head/450093/57 "2008-05-23T17:24:25Z")
My dad has an old set of books called something like “The history of mathematics” and there’s a section I remember reading as a kid, about mathematical prodigies including lightning calculators - people who can extract 20th roots of 15 digit numbers in their head, that kind of thing. They asked a very similar question as the OP, how one particular calculator squared a 12 digit number mentally in about 60 seconds. He said that it was the same as doing long multiplication on paper, he was just capable of running the numbers in his head very quickly without losing his place. So to do 365\*365 he’d mentally add up: 300_300+300_65+300_5+65_300+65_60+65_5+5_300+5_60+5\*5 Nothing amazing about the method, it’s just the sheer speed and ability to store the parts of the solution without getting them confused that are incredible. It wasn’t “I don’t know, the answer just comes to me”. ---
**Author:** ![Bearflag70](https://avatars.discourse-cdn.com/v4/letter/b/8e7dd6/32.png) [@Bearflag70](https://boards.straightdope.com/u/Bearflag70)\ **Post date:** [May 23, 2008, 5:38pm UTC](https://boards.straightdope.com/t/math-in-your-head/450093/58 "2008-05-23T17:38:26Z")
I just work the numbers from left to right. 1. 300 + 300 = 600 2. 60 + 60 is more than 100, so the result will be 700-something 3. 60 + 60 = 120, so the answer will be 720-ish 4. 5 + 5 = 10, so the answer is 730 Same process for something like 842 + 576: 1. 8 + 5 = 13, so the answer will be 1300-ish 2. 7 + 4 = greater than 10, so the answer is 1400-something 3. 7 + 4 = 11, so the answer is 1410-ish 4. 6 + 2 = 8, so the answer is 1418 [Previous page](https://boards.straightdope.com/t/math-in-your-head/450093.md?page=2)