Mental Multiplication Math Trick - Multiply numbers in your head near 100 - Free Educational videos for Students in K-12

Mental Multiplication Math Trick - Multiply numbers in your head near 100 - By tecmath

Transcript


00:00	Welcome to the Tech mouth channel . What we're going
00:02	to be having a look at this video is a
00:04	really , really rapid way of mental multiplication called the
00:08	base method modification . And this time we're looking around
00:10	numbers around 100 . So in previous videos have looked
00:13	through numbers around tens , twenties thirties , fifties .
00:16	Okay . And where we might be multiplying , say
00:19	37 times 53 how we might deal with that sort
00:22	of thing in a mental rapid way of actually multiplication
00:27	. But this now we're looking at A later version
00:30	which is looking at now numbers near 100 . And
00:33	the good news is as probably one of the easier
00:36	sorts of multiplication methods . Now , if you haven't
00:40	seen the previous videos , I really recommend you do
00:43	. Okay , I'll put the links up . It's
00:45	up to you . So What is based multiplication based
00:49	multiplication is where when multiplying two numbers , we use
00:52	an easier number to help us . So with 100
00:56	basement of multiplication , they're going to be using the
00:58	base of 100 . Okay so we might use this
01:01	say we're doing multiplication of say numbers like 100 and
01:05	five times 100 and seven . Okay . Yeah .
01:11	A number that's really easy . Near this is 100
01:14	and hundreds were really really basic number to multiply would
01:17	. So how do we use this base ? First
01:19	off ? What we do is we see how far
01:21	our number is from the base . So this number
01:24	is five more . So we write that as plus
01:27	five and this 177 . More than 100 . Okay
01:31	you go with that . Okay . So what we
01:33	then do is we then cross add what we've been
01:36	doing in these other videos . Okay so 100 and
01:39	five plus seven or 100 and seven plus five .
01:42	The answer is going to be the same . So
01:44	we get 100 and 12 . And then what we
01:47	do is we multiply this number by a base near
01:50	hundreds of really easy number to multiply by Yeah ,
01:53	it's a matter of just putting two zeros on the
01:54	end . 00 11,200 . Okay . Then what we
01:59	do to get our last part of the answer is
02:02	we multiply these two numbers here . Okay . So
02:06	it's seven times five Plus seven times plus five .
02:10	It's 35 . Okay . It's plus 35 . We're
02:13	going to add 35 to our answer . So where
02:16	we do this , we get the answer 11,200 and
02:21	35 . And you don't notice this is actually quite
02:24	similar to another video I made also a couple of
02:26	years back . Actually , it's a similar sort of
02:29	idea to this works on the same principles . So
02:32	did you understand all that ? So let's do another
02:36	example of this . Okay , so um let's do
02:42	Using again based 100 let's say do 109 . And
02:49	we're gonna times this by 102 . Okay , so
02:55	this year is nine more than 100 . This year
02:59	is two . More than 100 . Okay . We
03:02	cross add so 109 plus two or 102 plus nine
03:07	. We get the answer of 111 Times in by
03:12	100 . There's no answer because the times when the
03:14	base we get 11,100 and then we multiply this these
03:19	two numbers in the last part of Iran to nine
03:21	times two is 18 . So answer 111 11,100 and
03:29	I don't How did you go with that ? Pretty
03:32	good . You're gonna notice by the way you can
03:34	actually get away with often not putting these zeros down
03:37	and just writing this number straight . And if you
03:39	if you're doing it written and I'm encouraging you to
03:41	do this mentally now . Okay . Because that's what
03:44	this base number of multiplication is more about is trying
03:46	to start to do things mentally . Okay let's are
03:51	have a look at another example . What about we
03:55	do ? Okay 106 times 97 . Again we're going
04:02	to use base multiplication . 100 . We're gonna use
04:05	the base of 100 . These numbers are both near
04:07	100 . So this one is six more than 100
04:11	. And this one as you might notice Is less
04:14	it's -3 , that's three . Less than 100 .
04:18	So we do the same process though . Okay ,
04:20	106 minus three or 97 plus six . Whichever one
04:25	you prefer , you're going to get the answer of
04:27	100 and three . Okay , with R00 actually it's
04:31	going to leave those two blank spaces . Okay then
04:35	what we do is we multiply these two numbers together
04:40	. Okay . six times -3 is -18 . So
04:46	we're gonna take 18 off our answer . Okay ?
04:49	So maybe I should actually left these zeros in here
04:52	. Yeah . Let's take 18 off . 100 and
04:57	10,300 . We're going to get the answer of 102
05:04	eight two . Okay . 10,282 . Did you go
05:11	okay with that 1 ? All right . Let's have
05:15	now look a bigger example on a um So I
05:22	do What about we do this one ? 100 and
05:26	12 and we're times this by 100 and no .
05:32	Okay . Really quickly . This is 12 more .
05:35	This one here is nine more . Okay . 112
05:40	plus nine . We're going to get the answer of
05:42	100 and 21 . Yeah . 12,100 . Okay .
05:48	You're going OK so far then what we do is
05:52	nine times 12 hopefully you know you nine times 12
05:55	which is 100 and eight . We're going to add
05:57	that to our answer . We get the answer .
05:59	12,208 . Okay . I wanna be able to give
06:04	you a bunch of examples . You can pause the
06:06	video , you can try to answer them . Okay
06:11	? Let's give you a couple of examples . What
06:13	about 100 and five times 100 and one . What
06:18	about uh 97 times 90 three . And what about
06:26	92 times 112 . Okay , pause the video .
06:36	See here you go with these . So how did
06:39	you go ? I think I actually gave you examples
06:41	. Much harder than I did . That's the way
06:43	I do think it's OK more fun to get your
06:45	door the work . Yeah . So this one's Five
06:48	more because we're using base 100 . This one's five
06:50	more . This one's one more . 105 Plus one
06:56	. It's 106 00 And then we have five times
07:02	one which is five . So I'm gonna add five
07:04	to our answer and we're gonna get the answer of
07:07	10,600 and five . Okay . The next one this
07:13	week is in base 100 again , this is three
07:15	less . This is seven less . Okay , So
07:20	despite tidy that up a little bit , this one
07:22	here is seven less . So what do we get
07:27	? We get 97 takes seven . We're going to
07:29	get the answer of Dottie 9003 times by the base
07:36	we get 9000 and then seven threes at 21 .
07:41	So we're gonna add 21 because minus times minus is
07:44	a positive answer . We're going to add 21 to
07:46	our answer . We get the answer . 9,021 .
07:52	Okay . What about this 1 ? We are eight
07:58	were using base 100 again . We're eight below 100
08:01	and were 12 above 100 . So let's do the
08:06	same thing we've been doing the whole time . We're
08:09	going to get 100 and 12 tecate we get 100
08:12	and four . Okay , So get 10,400 And then
08:18	eight times 12 is 96 but it's not eight times
08:21	12 . It's -8 times 12 sides -96 . Okay
08:27	. What did you get is your answer ? Hopefully
08:29	you got 10,000 300 at four . Did you go
08:38	good with those ? Hopefully you did . I'll tell
08:40	you what , I'll put the links up in a
08:42	little bit for all those other videos . Just just
08:46	to finish this off . I reckon I'm just going
08:48	to put a bunch of of all the other ones
08:50	we've done so far up . So some examples of
08:52	these . Okay , so see how you go with
08:55	these , and I think it would be a good
08:58	little way of finishing off this particular idea . Okay
09:03	, what about we do 21 Times 14 . What
09:09	about 36 times uh 29 . What about 55 times
09:22	uh 56 and 104 times 97 . And these are
09:30	the types of things we've been doing in all these
09:31	other videos . Yeah , pause it , see how
09:34	you go . And if you can't do these ones
09:36	, you might want to go back and have a
09:37	look at these other videos . Yeah . Okay .
09:41	Let's have a look . So hopefully what you got
09:44	was this the first one we're going to use a
09:45	base of . I'm going to use a base of
09:47	20 . Okay , so this one was one more
09:50	than 20 . This one here was six . Less
09:54	than 20 . So 14 plus one We get 15
09:58	. But you remember we had two times by the
10:00	base , so 15 Turns . 250 . Okay .
10:08	And then I actually might write that in a different
10:11	color . They pointed actually changing the color scheme I've
10:14	been doing so 300 . And then what we do
10:17	is we get uh -6 times one is -6 .
10:21	We're going to take six off our answer . You
10:24	get the answer . 294 . Okay . What about
10:28	this next one ? What base did you use for
10:29	this one ? Hopefully I would use 30 this 16
10:33	More than 30 . This one here is One less
10:37	than 30 . Okay , let's have a look .
10:41	Uh 36 . Take one is 35 . Did you
10:45	get that ? 35 . So 35 when we had
10:48	half the times that by the base of 35 ,
10:51	Times three is 105 . But it's times 30 so
10:55	it's it's 1050 . And these two numbers we multiply
11:02	we get minus one times minus six , so we're
11:04	going to take six off our answer once again and
11:08	we get the answer of 1000 and 44 . Okay
11:17	let's have a look at this next one . So
11:19	We're using base 50 . This times where five above
11:22	50 where six above 50 . Okay so 56 plus
11:28	five We get 61 . Okay did you get that
11:32	61 ? If you remember what we did with this
11:34	is we times to buy 100 . So we got
11:37	6100 and then we halved it . So we've got
11:40	3000 and 50 . Okay ? Because they were multiplying
11:45	by 50 there And what we then get is six
11:49	times five is 30 . So we're gonna add 30
11:52	to our answer . We get the answer of 3090
11:58	. How'd you go with that ? Good ? All
11:59	right . Last one . This one should be really
12:02	really good because we've just done this before . Uh
12:04	this one's we're using base 100 . I've already started
12:07	doing without even thinking we're four . More than 100
12:10	. Where three less than 100 104 take three is
12:14	100 one . Okay . Time's up by 100 .
12:21	We get 10,100 . And then what we do is
12:24	we multiply these two numbers four times minus three minus
12:28	12 . Hey let's just take 12 straight off our
12:30	answer . We're gonna get the answer . 10,000 and
12:34	what are we taking off ? 12 of . So
12:37	I thi right , you go right with those .
12:42	If not , here's the links to the various sections
12:44	up now or where the relevant parts are . I
12:47	hope you really , really like that little series .
12:49	I find it a great little our method anyway .
12:52	Thanks for watching . See you next time .

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