Fast Math Trick - How to multiply numbers under twenty faster than a calculator - By tecmath
Transcript
| 00:0-1 | Good day . Welcome to Tech Mouth channel . Look | |
| 00:01 | at one of my favorite math calculation tricks today , | |
| 00:03 | where I'll show you how to multiply two numbers in | |
| 00:06 | their teens instantly . That's a nice , easy trick | |
| 00:08 | to learn . And I'll have you smashing out these | |
| 00:10 | calculations and putting the wind right up your teacher . | |
| 00:13 | A quick smart alright , here goes . And you | |
| 00:15 | got three seconds to answer this . Do remember this | |
| 00:17 | is a mental trick . So do try to do | |
| 00:19 | this in your head . So the first example here | |
| 00:21 | is 14 times 12 . Now time's up . Did | |
| 00:26 | you get the answer of 168 If you do , | |
| 00:30 | congratulations . If not , I'm going to show you | |
| 00:32 | how to do this in your head . Okay . | |
| 00:35 | And it's fairly simple . All right . I'm gonna | |
| 00:36 | get rid of the answer here and I'll show you | |
| 00:38 | on my thought processes as we do this . The | |
| 00:40 | first thing we do is I look at both the | |
| 00:43 | units numbers for our 14 and 12 here , so | |
| 00:45 | we have a units number before and for the 12 | |
| 00:48 | we have a units number over two . The next | |
| 00:51 | thing I do is I would add one of these | |
| 00:54 | , the two either head of the 14 or the | |
| 00:56 | four to the 12 . It gives us the same | |
| 00:57 | answer . So say 12-plus 4 . This 16 I | |
| 01:01 | think this is our tens number . So I think | |
| 01:03 | this is 160 . Okay , 16 becomes 160 . | |
| 01:08 | The second thing I do is for our units part | |
| 01:10 | is I would multiply these two numbers . Okay , | |
| 01:13 | so four times two is eight . And then what | |
| 01:17 | I do is I just add these guys together . | |
| 01:19 | So what we have is 160 plus eight . Really | |
| 01:23 | easy to do in your head is 100 and 68 | |
| 01:26 | . All right , pretty simple . Right , Okay | |
| 01:30 | . What about we go through another example . You | |
| 01:32 | probably notice with this as well . You can do | |
| 01:34 | these in your head . You do not need to | |
| 01:35 | be writing these down . So I'm writing a little | |
| 01:37 | bit extra when I do this . Okay , so | |
| 01:39 | let's go through another harder example now and see how | |
| 01:43 | you go . I'm going to give you five seconds | |
| 01:44 | to answer this one . Um And What about we | |
| 01:48 | give this one to go ? 16 time 17 . | |
| 01:55 | All right . Okay . I reckon five seconds is | |
| 01:57 | up . So the very very first thing I do | |
| 02:00 | is I look at the units numbers we have a | |
| 02:02 | six and we have a seven . The next thing | |
| 02:05 | to do is I'm going to add either the seventh | |
| 02:07 | . I'm just going to add is seven to the | |
| 02:08 | 16 here , Which is 23 which has a tens | |
| 02:11 | number becomes 230 . All right . 6 7 42 | |
| 02:16 | . So 230 plus 42 . We get our answer | |
| 02:20 | of 272 . Like I said , really , really | |
| 02:25 | simple . Okay . What about I give you another | |
| 02:30 | example and then what we'll do is we can go | |
| 02:31 | through and look at why this particular trick works . | |
| 02:34 | Okay . So all right . What about 15 times | |
| 02:38 | 19 ? Okay . We have a five . We | |
| 02:43 | have a 99 plus 15 is 24 . So we | |
| 02:47 | get 240 . Is that tens number that we're starting | |
| 02:50 | in our head ? Five nines are 45 . So | |
| 02:53 | 240 plus 45 is 285 . And that is the | |
| 03:00 | how you multiply numbers in their teens with one another | |
| 03:04 | really , really easily in your head . Okay . | |
| 03:06 | So why does this work ? Let me show you | |
| 03:08 | . Okay , so first off , you're gonna think | |
| 03:10 | about these numbers here . I'll just give you an | |
| 03:12 | example . Say we were multiplying 12 times 18 , | |
| 03:17 | okay ? And you can probably work this out in | |
| 03:19 | your head , but I just want to show you | |
| 03:20 | what's going on here . I'm going to think about | |
| 03:22 | this being a number 10 plus A . In this | |
| 03:26 | case 10 plus A is equal to two . Okay | |
| 03:29 | ? And we're multiplying this by 10 plus B . | |
| 03:32 | In this case B is equal to eight . But | |
| 03:35 | these numbers can obviously vary . All right . So | |
| 03:38 | , we were to multiply this out . Okay . | |
| 03:40 | The first thing we get is 10 times 10 , | |
| 03:42 | Which is 100 . Okay ? 10 times a . | |
| 03:46 | Which is 10 a . We have to that we're | |
| 03:49 | gonna add 10 times b . And to that A | |
| 03:53 | times B . All right . So , we expand | |
| 03:55 | those brackets out . This is what we get . | |
| 03:58 | All right . So , what we can do now | |
| 04:00 | is we could Basically divide this number by 10 . | |
| 04:03 | We could put this number in brackets by 10 . | |
| 04:05 | I'll show you what I mean . This these three | |
| 04:07 | numbers all have 10 going to it . So , | |
| 04:09 | we can basically are shortness out . We can say | |
| 04:11 | this is 10 times 10 plus A plus B plus | |
| 04:17 | a B . You see what I did there ? | |
| 04:19 | Because 10 times 10 is 110 times A is 10 | |
| 04:22 | 8 . 10 times B is 10 B . And | |
| 04:24 | we have the baby . Now , what's going on | |
| 04:26 | here ? And let's put this in terms of about | |
| 04:28 | 12 times 18 here . Okay . So , the | |
| 04:30 | very first thing we have is we could think about | |
| 04:32 | this . Yeah , A&B is two and 8 . | |
| 04:35 | Okay . The very first thing we have is say | |
| 04:38 | 12 plus eight . You know when we say , | |
| 04:39 | okay , this is 12 and they were basically gained | |
| 04:42 | 10 plus two plus eight . Okay , So 12 | |
| 04:44 | plus 18 for instance , is 20 . All right | |
| 04:47 | . So I'll put this over here . The 20 | |
| 04:49 | becomes 200 . Okay , the next thing we have | |
| 04:53 | is a B . Here . Okay . Because we | |
| 04:54 | multiply that by 10 . The next thing you have | |
| 04:56 | is a B . A . Times B is two | |
| 04:59 | times 83 times eight . Is that part 16 ? | |
| 05:02 | 216 . Yeah . So that's why this particular trick | |
| 05:06 | works . Okay . That's a proof of how it | |
| 05:09 | works . Anyway , if you like this video , | |
| 05:12 | please remember to like and subscribe . Hopefully you do | |
| 05:15 | like it . It's a really , really simple trick | |
| 05:16 | in . Hey , just a big shout out to | |
| 05:18 | my latest patron . Dave . Thanks . Thanks . | |
| 05:21 | Thanks . Thanks . Thanks . You patrons are being | |
| 05:24 | really , really great . Okay . You're helping keep | |
| 05:26 | the mats free and that's a really awesome thing . | |
| 05:28 | So anyway , really making a few more of these | |
| 05:30 | math trick videos . I'm updating a few of them | |
| 05:32 | . I have a few that are sitting pretty old | |
| 05:34 | , but the sound quality is a bit rubbish and | |
| 05:36 | things like that . So hopefully we can fix those | |
| 05:39 | up , make the sound quality bit better . Put | |
| 05:40 | him in HD and uh , you know , engage | |
| 05:43 | your brand new audience in loving maths . Anyway , | |
| 05:46 | thanks for watching . We'll see you next time . | |
| 05:48 | Bye . |
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