Fast Math Trick to subtacting numbers mentally - two digit numbers - By tecmath
Transcript
00:0-1 | Okay , Welcome to Tech Math Channel . What we're | |
00:02 | gonna be having a look at this video is we're | |
00:03 | going to be looking at methods for mental subtraction . | |
00:06 | Okay , so it's going to start out fairly easy | |
00:09 | . We're gonna start with some two digit numbers . | |
00:11 | We're gonna get to some harder problems as we go | |
00:13 | along though . Now there's a couple of different methods | |
00:16 | you would be using when you are doing mental subtraction | |
00:19 | . Typically both these methods go from left to right | |
00:21 | . You'll see what I mean by this in a | |
00:23 | minute . But you're going to deploy the different methods | |
00:25 | just for different types of subtraction questions . One where | |
00:29 | you're not borrowing questions and one where you are trying | |
00:32 | to borrow off other numbers . You again , see | |
00:34 | what I mean with this . So an example of | |
00:37 | the type of questions that you might get when you | |
00:39 | asked to subtract one number and I'll give it a | |
00:41 | two digit number uh say we had 90 eight and | |
00:48 | I wanted to take away 23 . Okay , so | |
00:51 | this is a nice easy one . Now , the | |
00:56 | way that I would guess that most people would do | |
00:58 | this mentally is as follows , They would be seeing | |
01:01 | 23 as two different doubles . I'll be seeing it | |
01:04 | as 20 And three . And if this is not | |
01:07 | the way you do it , this is not a | |
01:08 | bad little technique anyway . So you are taking away | |
01:10 | 20 and then you're taking away three from 98-98 . | |
01:14 | Take away 20 78 . Take away three is 70 | |
01:19 | five . Okay , so what about I give you | |
01:24 | another example of this ? Okay , so there's a | |
01:27 | nice easy method of doing these to start off with | |
01:29 | nice weather . We don't have to borrow say we | |
01:32 | had to say something like uh 68 and we're going | |
01:38 | to take away 20 five . Okay . So nice | |
01:43 | and easy . So 68 is made up of 20 | |
01:46 | and 68 . We have 25 were taking off to | |
01:49 | 68 . 48 . Taking off from 20 40 three | |
01:57 | . Okay . So he took off the 21st , | |
02:02 | we got 48 that we took on the five and | |
02:04 | we've got 43 . How'd you go with that ? | |
02:06 | Okay . They're they're fairly easy questions . These are | |
02:10 | really basic ones where you don't have to go about | |
02:12 | borrowing and I'll show you what I mean by this | |
02:14 | this idea of borrowing . Because I think this is | |
02:16 | where it gets tricky with mental subtraction . Occasionally you're | |
02:20 | going to start well quite often you're gonna start getting | |
02:23 | numbers like this one you subtract , you're gonna start | |
02:25 | getting numbers say like uh what about we do something | |
02:29 | like 84 and we take away 59 . So if | |
02:37 | we were to do this you're gonna see straight away | |
02:39 | that it's a little bit hardest . Could go 84 | |
02:41 | and take off 50 get 34 and then take away | |
02:44 | the dive And you would get 25 . But there's | |
02:48 | also another way that you could do this which you | |
02:51 | might find a little bit easier . Okay ? You | |
02:54 | don't have to do any sort of mental acrobatics in | |
02:58 | your head . Well , not as much anyway , | |
02:59 | you turned into a bit of an additional problem and | |
03:02 | this is the way you do this as follows . | |
03:04 | So 59 we're going to round it up to 60 | |
03:07 | . Okay ? And you're probably gonna realize that we | |
03:10 | have taken one too many here . Okay ? We | |
03:12 | take off 60 different is one . So we have | |
03:15 | 84 . Take away 60 , We have 24 . | |
03:20 | Okay ? But We were supposed to have 60 . | |
03:23 | Was supposed to have $50 . I was taken off | |
03:25 | 1-2 . Men have 1-2 . Too many . So | |
03:29 | we're gonna have to add that one back hold . | |
03:30 | So the answer is 25 . Okay , So we | |
03:36 | take off that next time we're up . So 84 | |
03:39 | take away 60 is 24 plus one . So I | |
03:41 | started into a bit of an addition problem . So | |
03:44 | I'll give you a tough war examples of these . | |
03:50 | So say we had a question , uh , 82 | |
03:54 | and we are going to take away 34 . So | |
04:00 | what you won't do for this Is you might round | |
04:03 | this up to 40 . You're going to realize that | |
04:06 | we've taken six too much . So 82 42 48 | |
04:13 | . Okay , I'll show you how I've done this | |
04:15 | . Okay ? So we've got six too much , | |
04:17 | right ? That little six down there . So 80 | |
04:19 | to take away 40 . It's 42 and I'm gonna | |
04:23 | add this six or which is 48 . And the | |
04:28 | Beatrix for these is not so much all this writing | |
04:31 | you see here . Because when you think , Hey | |
04:32 | , that's even slower . It's trying to do this | |
04:34 | in your head . Okay , This is the trick | |
04:36 | . This is mental . Uh subtraction , we're after | |
04:38 | here , not a written subtraction . It's always hard | |
04:41 | to try and explain mental subtraction . Sometimes you're writing | |
04:44 | at the end at the same time . So I'm | |
04:46 | gonna give you a bunch of questions where they say | |
04:48 | we did uh What about 67 take 48 . You | |
04:56 | might get grants of this really , really quickly . | |
04:58 | And what about we do 51 Take away 27 . | |
05:06 | Okay . And we will also do 148 and we'll | |
05:13 | take away 70 . Doing Okay . Three different questions | |
05:23 | . Okay , so how we're going to get these | |
05:26 | ? So first off , I'm going to do this | |
05:27 | mentally now . Okay , so 67 take away 50 | |
05:33 | is 17 plus two . His daughter , 51 Takeaway | |
05:40 | 30 is 21 plus three is 24 . 148 . | |
05:50 | Take away 80 is 68 plus one . The $60 | |
05:56 | . See how easy that is . Okay , what | |
05:59 | about to give you a couple ? It is just | |
06:01 | a little bit harder . Okay , what about we | |
06:05 | do this type of one ? What about we do | |
06:08 | ? Say something like 160 two and I'm going to | |
06:14 | take away uh Yeah . 37 . Okay . That's | |
06:23 | not that hard at all , is it ? Actually | |
06:25 | ? That's a really easy one , because this is | |
06:26 | almost 100 take that off . It's going to be | |
06:28 | 62 plus three . Our answer is going to be | |
06:31 | 60 five . Okay , So round that up to | |
06:35 | 100 , we took that off , We've got 60 | |
06:38 | to that . We added the three extra . We | |
06:40 | need we got 65 . Do you see how easy | |
06:42 | that is ? Okay in the next video , what | |
06:44 | we're gonna be having to look at is were to | |
06:45 | extend this into bigger numbers . So hopefully you find | |
06:48 | this handy . Uh and if you do look at | |
06:51 | this next video , we'll take this even further anyway | |
06:54 | . Thanks for watching . See you next time . | |
06:55 | Bye . |
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Fast Math Trick to subtacting numbers mentally - two digit numbers is a free educational video by tecmath.
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