How to Multiply Numbers Lightning Fast like Shakuntala Devi (Mental Maths #4)

you must have heard of shakuntala devi she made a world record by multiplying this and this to get this in guess what how much seconds 20 seconds with the method known as crisscross method that we will learn in this video but i just want to tell you that even with knowing this method doing this is super human because in doing this she had to do 169 multiplication problem and 167 addition problems a total of 336 operations in just 20 seconds that means one operation in one tenth of a second and also she wrote the last number which is just superhuman even the reaction time is also superhuman i told you this because i want to show you the highest possibilities of learning mental maths so with this let's get started [Music] okay so before starting this video i just wanna tell you that i am gonna throw a lot at you in this video so make sure that you have watched the part one part two and part 3 of this mental math series because the secrets to mental maths book have most of its chapters on multiplication and i'm gonna share with you the best things which i found in those chapters okay so in the part 1 of this series i showed you how to square any two digit number ending in five lightning fast now let's learn how to square any two digit number okay so let's learn how to square 13 mentally now it's very simple you either need to round the number up or down now 13 is closer to 10 so i will round it down so 10 now we have subtracted 3 from 13 so add 3 in 13 so 16 now we have 10 and 16 now multiply these two so 16 times 10 is 160 which is very simple now in this 160 you just need to add the square of 3 which we have subtracted so 160 plus 9 is 169 which is the square of 13 and this how you can square any two digit number now try 12 so round down 10 multiply by 14 140 and 2 square is 4 so 144 is our answer so this is how to square any two digit number now let's try 19 this is a little bit difficult because there will be a 20 here okay but don't worry so 19 round up 20 one round down 18 so 18 time 20 now 18 twos is 36 so 360. now in here we just need to add 1 square which is 1 so 361 is our answer so see you have just a 19 square that is 19 times 19 in your head right now in here now you can take any number from 11 to 19 in here you can go further but it would be a little bit difficult for that you need to learn two by one multiplication so let's learn two by one multiplication which is very simple same method here as we did in addition and subtraction always go from left to right and if you have watched those videos you will know what is its benefits so let's multiply 43 and eight now instead of multiplying three with it then taking care then four with it and then adding we will go the easier way the left to right method so 40 times 8 first so 40 times 8 is it was 32 so 320 now 3 times 8 which is 24 so 320 plus 24 is 344 so this is the basic two by one multiplication so in this problem we converted 43 into 40 plus three this is known as the addition method for multiplication the other method is the subtraction method it is used for problems like this 59 times 6 now instead of going like 50 plus 9 we will do this 60 minus 1 times 6 so 60 times 6 is what 6 6 are 36 so 360 minus 6 so 360 minus 6 is 354 this is our answer see this is a really big problem you have just done mentally in here now after learning this two by one multiplication you have also learned something else the square of any two digit number so for example 83 square now round down 80 times 86 now this 80 times 86 seems 2 by 2 multiplication but it is just 86 multiply by 8 with the 0 you can do this problem either by addition method or subtraction method it's your choice i will go with addition method so 86 times 8 so 80 plus 6 times 8 so 88z 64 640 plus 86 of 48 so 640 plus 48 so 640 plus 48 is 688 and we have a zero also so six eight eight zero so adding just three square in here which is six eight eight nine which is the square of 83 so see if you are able to do this you have just done the square of 83 in your head which is insane now let's go with 2 by 2 multiplication now 2 by 2 multiplication is really interesting because there is the addition method of multiplication there is the subtraction method of multiplication and there is also another method known as factoring method also you can also mix these method together to get creative and also remember that the more creative you are the faster you will be so let's take one problem and do it with all three addition subtraction and factoring method after that the more you will practice the better you will become and also in 2 by 2 multiplication the factoring method is the best you will see that in this problem so 46 multiplied by 42 now think most of people can't even do this mentally but here we are going to do it in three different ways now before doing this i just want to tell you the rule of thumb with which you can store numbers in your hands which would be really helpful in doing these two by two three by three extra problems this is extremely simple so this is one this is two this is three this is four this is five six seven eight and nine you can store in this hand too so you can store two numbers in your hands have you ever seen those videos in which students are doing like this this this this and doing mental calculation they are doing this same method but a little advanced one with their hands they are mimicking the abakas they are also storing numbers in their hand but they are going 200 000 so that is a little advanced method and story of another video so the addition method for 46 times 42 now 40 plus 2 times 46 now store 2 in your hand like this okay so 40 times 46 which is just a 2 by 1 multiplication so 46 times 4 so 4 for the 16 160 plus 24 184 so 1 8 4 now one eight four zero plus forty six times two which is ninety two so one eight four zero plus ninety two which is one nine three two this is our answer so this how we can use addition method to solve problems but this was not that easy there were a lot of small problems that we had to do in this so let's try the subtraction method for 446 times 42 so 50 minus 4 times 42 4 in our hand like this so 50 times 42 which is 5 times 42 with a zero so what is 5 times 42 five fours are 20 5 to the 10 210 with a zero so two one zero zero now two one zero zero minus four times forty two so what is four times forty two so four for the sixteen photos are eight one 168 so 2 1 0 0 minus 168 now in here we subtract 100 first this is a basic subtraction problem so 2 1 0 0 minus 100 is what 2 000 minus 68 now in here we can be very cool with the complement of 68 which is 32 so the answer is 1932 so this was the subtraction method for multiplication of 2 by 2. so this was easy but not that easy so let's see the factoring method for multiplication of two two digit numbers now the factoring method is a really easy method but it works only on numbers which can be factored like we can write 42 as 6 times 7 and then also we can write 6 as 3 times 2 and this will make our multiplication really easy so let's apply this factoring method on 46 times 42 so 46 times 42 can be written as 46 times 6 times 7 we can also write 6 as 3 times 2 but let's not do that because we know 2 1 multiplication very well so let's store 7 in our hand so 5 6 7 so seven here so we have 46 times six so six fours are 24 240 plus six sigma 36 so 240 plus 36 which is 276. now we have 276 times 7 so 276 times 7 this is a 3 2 1 multiplication so let's do it 276 times 7 so 7 to the 14 so 1400 plus 7 7 of 49 1400 plus 49 and there is a zero also so 1400 plus 490 which is one eight nine zero okay one eight nine zero now you have to say it again again one eight nine zero one eight nine zero so one eight nine zero plus there is a seven times six so seven six are 42 so 1 8 9 0 plus 42 so 1 9 3 2 so this is our answer 1932 is our answer now this factoring method was way easier than addition method and subtraction method but this also depends on the problem that we have sometimes subtraction method works best sometime addition and sub time factoring method so it depends on you that which one you want to use and you will understand it automatically the more you practice and for practicing i have specially made a game named math blob run in this game there is a good blow when there is a bad globe chasing that blob who wants to eat it and solving math questions the mental math questions will save your blob so this game will help you to practice mental math downlink of this game is in the description of this video now let's learn that legendary crisscross method for this you need a pen and paper but don't worry your mind for dressed when in this book i was going to learn this method i thought that after learning this i will be like shakunsala devi i was able to solve long multiplication problems really fast but just wanna tell you this method is nothing special it is just a different way to see multiplication problems but it is way easier than the method they teach us in school but on the other hand it also won't make you a superhuman calculator with practice you can get better at it like shakuntala devi so let's learn the crisscross method so 47 times 34 using the crisscross method so write 47 times 34 on paper like this now draw a line like this so seven fours are twenty eight write eight in here you can also write the two on paper or you can store it in your hand like this now draw a diagonal like this so four fours are 16 and in this 16 add this 2 so 16 plus 2 is 18 now 7 3's are 21 so 21 plus 18 now you can do this mentally or write it on paper i will prefer to do this mentally so 21 plus 18 is 21 plus 10 which is 31 31 plus 8 is 39 so write 9 in here and we have a 3 now store three in your hand or you can write it on paper like this now this one four times three so four times three is twelve and this twelve add three which is fifteen so one five nine eight is our answer so this is basically all the criss cross method that you need to learn now let's try it on a three cross three problem so 853 times 762 using the legendary crisscross method of pen and paper so write them like this 853 times 762. now three twos are six no carry five twos are 10 plus six threes are 18 10 plus 18 28 8 in here and two in your hand like this now you need to take a longer diagonal so a 2s are 16 16 plus 2 18 18 plus now center six fives are 30 30 plus that 18 which is 48 now remember 48 48 plus seven threes are 21 48 plus 21 is what 48 plus 20 68 68 plus 169 so nine in here like this and six in your hand like this or on paper whatever you choose now eight six are 48 48 plus six is 54 54 plus seven fives are 35 now 54 plus 35 this is a longer addition problem 54 plus 30 is 84 84 plus 5 89 so 9 in here or you can store 8 like this 5 6 7 8 now 18 here so 7 8 are 56 56 plus 8 is worth 64. so this is our answer 64986 so congrats you have just done the three cross three multiplication problem using the legendary crisscross method this method is basically converting a long multiplication problem in simple multiplication and addition problems so think what shakuntala devi had to do in those 20 seconds so these were all the real ways with which you can perform lightning fast multiplication now in the next video we will attack on titans that means divisions so till then practice a lot and always remember that math is everything

Channel: mathOgenius

Views: 816,374

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Id: DV4YgX0iU00

Length: 15min 9sec (909 seconds)

Published: Thu Jan 14 2021