Fast Math Trick to subtacting numbers mentally - two digit numbers - Free Educational videos for Students in K-12 | Lumos Learning

Fast Math Trick to subtacting numbers mentally - two digit numbers - Free Educational videos for Students in k-12


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 .
Summarizer

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