Fast Mental Multiplication Trick - multiply in your head using base 10

g'day and welcome to the tech math Channel this video is the first video in a series of videos where we're going to be having a look at the base method of multiplication it's a method of multiplication multiplying bigger numbers where you can start to mentally multiply bigger numbers using this idea of a base now what is this it's a base is an easy method of multiplication we're basically what you're doing is you're multiplying two numbers that we're gonna start out of they're gonna be fairly close to one another but they only get further and further away we're gonna multiply these two numbers using a third number a nice easy number which we call the base and you'll see how this works so a few you guys will be able to multiply some of these numbers quite easy we've seen my earlier videos but this videos was just starting this method of multiplication which we will build upon so it's really really worth sitting through and watching cuz you I do modify the way I can do this multiplication so first off we'll give you an example and if you don't do this fairly quickly what's 14 times 12 now look you pretty much automatically should get the answer come to your head that this is 168 okay if you didn't don't worry if you did congratulations but let me show you the way that we work using this basement now base number you're gonna say these numbers are quite close to one another first off they they do have a number which is fairly much in common and this number is a base number of 10 tens a number which is nice and close these numbers intense also a pretty easy number to multiply around with other numbers which we can use as bases you'll see in future videos will include things like 20 maybe 50 even 100 and they can go even further from there okay so these are nice easy numbers to multiply with then I'll show you how we play around with these base numbers so we're views were chosen the base number of 10 so when we look at 14 this 14 is four more plus four more than they're based on me so plus four and our 12 is plus two more then our base number 10 so the first way we get our answer is this we add although this number to this number so here through here to the four nine or we could add the 4 to the 12 there you see that so we we do a bit of cross addition sort of thing here and you're gonna say that it's gonna give us the same answer no matter what this is often the case this is actually always the case when we do this so you just have to choose one of them to add okay so first off let's add 14 to 2 here we get the answer of 16 yeah let's say we get our first part of our answer the next thing we do is we multiply these two numbers together okay this plus 4 and plus 2 okay and what do you get when you multiply plus 4 and plus 2 when you multiply 4 & 2 we get the number 8 so we get the answer of a hundred and sixty-eight okay you're pretty good with this okay so we can explore this a little bit more okay and I'll put another example up it's really really important you get this idea of first off which base we're using also how we're getting those numbers above those particular numbers so let's let's give you an answer here an answer let's give you a question you want we do this one 15 times 13 now again we're only gonna be playing around with the base 10 in this particular video but we will use other bases in other videos so using base 10 this is 5 above 10 and this is 3 above 10 so we add one of these numbers together with us so either this one or this one it doesn't matter again so 15 plus 3 we get 18 now look when you're multiplying these mentally I actually find it helpful to not think of this as a line but it's actually a hundred and eighty okay let's hear tens part so the addition part gives us our first part and then we might add these guys together so 5 times 3 is 15 and we're so we're gonna add these guys together so 180 plus 15 it's a nice easy one you can do in your head which is 195 okay I only write those they have only just so we can show what's going on I think these are really easy once to do in your head you see that okay I'll give you another example this is really critical that you get the basics of this if you decide to continue on with this so I wouldn't let me do another one we'll do 17 times 12 again we're using base 10 we're gonna go plus 7 plus 2 we add these two numbers together we get a hundred we get 19 which is we're seeing as a hundred and ninety we multiply these two numbers we get 14 190 plus 14 is 204 okay how'd you go with those pretty good I'll tell you what now that'd modify these a little bit so and you'll see this modification in a bit now again these some of these questions you may be able to answer fairly easy using different methods but I'm gonna encourage you just from the minute to use this particular method because it is gonna get trickier it's gonna get well some trickier so much of debilities gonna start looking at different paces so what do we do this one well there we go 8 times 17 because we can also you're gonna notice these numbers are close to 10 - so we can also use these numbers as well when we're using a base 10 but there's a little bit of a difference this time okay yeah this number here is 2 below okay I'm gonna use a different color to show that obviously in your head you'll you won't be able to do that so this is 2 below 10 and this one here is 7 above and again the same thing you can either 17 plus minus 2r 8 plus similar gives you the same answer of 15 or 150 then we multiply these guys together in our minus 7 that minus 2 times 7 is not 14 its minus 14 okay so we're gonna take 14 away from our answer and we get the answer of 136 okay you see how that worked these two guys give a negative number so we're actually taking our answer away here and it's a little trick that look it's easy to fall for occasionally when you are our first doing these okay what about I'm gonna give you I think an answer another question there you know the answer to almost straight away but let's investigate it anyway well maybe we're doing nine times nine now look hopefully you're looking at that straight away going yeah I know the answer to that and hopefully you're gonna say it's 81 but let's have a look at this using this method so this is one below ten this is one below ten so nine take one is eight so I'm gonna say there's 80 then minus 1 times minus 1 now minus times a minus gives us a positive number so you know get +1 okay 80 plus 1 is 81 cool yeah we can even go a little bit further than this we can we can actually do numbers almost on the other part of the spectrum I give you a couple examples in a second a couple of your own ones to work through we can even do something like say yeah this one 19 times 21 I can we can use this idea of bases here this number here is 9 above 10 this one here is I put a plus a this one here is 11 above 10 so 19 plus 11 is 30 becomes 300 and then 11 times 9 is 99 our answer you got it it's 399 but what you might notice with this is this numbers not only these numbers are not only sort of close to 10 but they're actually closer to 20 okay we can actually use base 20 to work these out and that's what we're gonna be having a look at in our next video is the idea of using base 20 okay now look I think I'll leave you with a few examples with this just to consolidate this but you just first off what you probably should get with this the sales video with this part of the video is decide you're using these numbers here this idea of using a base okay and how we deal with these when we are doing them okay cuz it actually doesn't vary a huge amount even when we start getting even bigger numbers or I'm gonna give you a couple of examples here see here you go of them what about we do these ones 14 times 13 what about we do I'll do one nine times 18 what about we do 15 times 50 okay pause the video and see here you go the answering these okay how'd you go let's have a look so this one's for above this one's a plus three so we get fourteen plus three which is 170 okay seventeen seventy one seventy and then we multiply these two numbers four times three is twelve so we add these guys together and we get the answer of 182 okay what about we do this next one now we get this one's eight above this one here nine is one below so nine plus 8 or 18 take one is 17 so again 170 and then we multiply these two guys one minus 1 times 8 is minus 8 okay so 170 minus 8 is 162 hey you going so far let's do this last one so this one's five above this one's five above we add these guys together so it's 15 plus 5 it's 20 becomes 200 five times five is 25 we get the answer of 225 are you gonna see that yeah this is all obviously on paper but these are not so bad to also start doing in your head okay the method the way you which you might do it might vary depending on how comfortable you are okay I sometimes you might do you might think well I'm gonna work at this time spit first so you keep the 8 in your head you know you're gonna take out off the answer and then your work out you know it's 170 take 8 okay then you get 162 it's a lot to do with this one okay anyway the next video we're gonna be having a look at to start looking this base - any idea so I hope to see you then bye you

Channel: tecmath

Views: 4,215,195

Rating: 4.7892733 out of 5

Keywords: mental, math, trick, multiplication, base, mental multiplication, math trick, menthal math

Id: Rgw9Ik5ZGaY

Length: 10min 50sec (650 seconds)

Published: Fri Feb 22 2013