Mental Math Multiplication - Instantly multiply with numbers ending in 5 - Free Educational videos for Students in K-12

Mental Math Multiplication - Instantly multiply with numbers ending in 5 - 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 we're
00:04	going to be having a look at how to multiply
00:06	a little trick of multiplying numbers that end in five
00:08	. It's a really fast trick And it's a really
00:12	a powerful trick when you get used to using it
00:14	. So I'll show you how to do this .
00:16	In fact , the type of question we're looking at
00:18	with these ones is as follows and say something I
00:20	want to ask you what's 30 for Times 15 .
00:24	So the answer to that , I reckon you should
00:26	have worked out by about now and hopefully answer you
00:29	got was 510 . Okay . And if you've got
00:33	510 , congratulations . If not , I'm going to
00:36	show you what people working these sort of questions out
00:39	almost instantly . Okay , so the way that we
00:42	do this is a bit of doubling , bit of
00:44	Harvey and I'll show you what I mean by this
00:45	. If we were to double this number here ,
00:48	the number that ends in five , we're going to
00:50	make that into a 30 , which is a nice
00:52	and easy number to play around a lot easier than
00:54	15 . We double this number . We have this
00:57	number Next week , you're gonna know it's an even
00:58	number having an odd numbers a little bit hard and
01:01	I'll show you how we deal with those a little
01:03	bit . Okay , so we have this number and
01:06	it becomes a 17 . This is like 17 times
01:09	three . And we're gonna put a zero on the
01:11	answer 17 times three . It's gonna be 30 plus
01:13	21 which is going to be 51 And then we
01:18	put zero on the end and that's how fast you
01:21	can do these . Okay , so as soon as
01:23	you see this , you double this number , you
01:25	have this number and it just multiplied straight out .
01:28	Really simple . Okay then that's the trick for do
01:31	we leave ? Okay , what about I'll give you
01:33	another one of these , you give this a go
01:35	. So what about 26 ? You're probably already looking
01:37	and thinking what's half of that time ? 45 .
01:41	So this is gonna be the same as yeah ,
01:44	half of this , which is 13 times 90 .
01:48	And we're going to get the answer to this ,
01:50	which is going to be 90-plus 21 , which is
01:52	gonna be 117 With a zero on the end .
01:57	Pretty cool . Right , okay . What about I'll
02:00	give you one more example where it's an even number
02:02	one if I'm going too fast or you can pause
02:04	me . As soon as you see there the question
02:06	there , you might pause me . Okay and then
02:07	see how you can go with this . So what
02:09	about we do 66 Okay . We're gonna be hardly
02:12	that aren't we ? Times are 15 , Which hopefully
02:17	you're well in front of me right now , this
02:19	doubled is gonna be 30 and this heart is going
02:21	to be 33 , so 33 times 30 is going
02:26	to be 33 times three is 99 0 on the
02:30	end . Hopefully you've got the answer of 990 .
02:32	Isn't that a really easy trick ? Yeah . Okay
02:35	. What about I'll show you what to do when
02:37	you get the problem of an odd number and it's
02:40	not much of a problem . Okay . Just we
02:43	do have to treat things just a little bit differently
02:46	. So sales to give you the question , what's
02:48	27 times 35 ? Okay . So first off obviously
02:55	Take care of this 27 , we know we're going
02:57	to double this here and get 70 and that's fair
02:58	enough . But what do we do here ? We're
02:59	gonna have it . But having this isn't so easy
03:02	. Okay . Okay . We can look at it
03:04	and say yeah , the half of this is 13.5
03:07	. But what we're going to do is I'm actually
03:10	just going to have the next number down . So
03:11	everyone to look at the next number down . The
03:13	next even number down the number below this just taking
03:15	off one is 26 . So by half-26 I get
03:20	13 and if I double this , I get 70
03:24	some of it Multiply these together and I'm just going
03:26	to add that extra one group of 35 on afterwards
03:30	. So after I get my answer here , I
03:31	just add this number on again . Okay ? Because
03:34	there's a group missing . All right , so 13
03:38	times seven is 91 . Okay , so that becomes
03:41	910 Plus 35 is 900 and 45 . Okay .
03:48	Because I placed on The Extra 35 . Okay ,
03:53	you're good with that . So what about we have
03:56	a look at one last one of these And we'll
03:59	see how you going is hopefully you're getting a clean
04:01	sweep and getting all these correct . What about we
04:04	just go 17 times 25 , 17 times 25 .
04:08	And what is this equal to ? So again ,
04:10	the next number down under this is 16 . Okay
04:13	, Because again this is stuff you can do in
04:15	nanoseconds . So I'm writing it down . It's taken
04:17	me a bit longer but you should be able to
04:19	do this a lot faster in your head . Okay
04:21	? So half of that is gonna be 88 times
04:24	50 , which is going to be 400 . And
04:26	then I'm gonna add on that extra 25 which is
04:30	going to give me the grand total Of 425 .
04:37	Okay , so how did you go with that ?
04:38	Was that good anyway ? Hopefully you like that trick
04:42	. It's a nice basic little trick anyway . Thanks
04:45	for watching . We'll see you next time . Bye
00:0-1	.

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