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Welcome to the community.
I hope you enjoy reading it. Feel free to ask any questions you have in the community.
I would recommend you use the Bridge or the Vitruvian man method for 3 digit multipliers to reduce the cognitive load.
UT Method is recommended for two digit multipliers. However it is still possible to do the UT method with the a 3 digit multiplier.
Here is the illustration of the Method
If you want to multiply 6671 × 386 here are the steps.
Step 1: Add the same number of zeros as the number of digits in the multiplier.
So the multiplier 386 has 3 digits. So we add three zeroes in front of 6671 to get 000671
0 0 0 6 6 7 1 x 386
Step 2: Sum of two pair products is a single digit of the answer.
Use your middle and forefinger to keep track of whether to calculate U or the T. Place each finger in the middle of two pairs.
Step 2.1
Using 6 in 386, the pair product of 0 and 0 is 0 + 0 = 0 (U6T6 = 00 + 00).
Using 8 in 386, the pair product of 0 and 0 is 0 + 0 = 0 (U8T8 = 00 + 00).
Using 3 in 386, the pair product of 0 and 6 is 0 + 1 = 1 (U3T3 = 00 + 18).
Adding the three number together we get 0 + 0 + 1 = 1.U6 T6
=Â Â U8 T8
= Â Â Â Â Â U3 T3
0Â Â 0Â Â Â 0Â Â 6Â Â Â 6Â Â Â 7Â Â 1 x 386 = 1 _ _ _ _ _ _Step 2.2
Moving to the next set of digits
Using 6 in 386, the pair product of 0 and 0 is 0 + 0 = 0 (U6T6 = 00 + 00).
Using 8 in 386, the pair product of 0 and 6 is 0 + 4 = 4 (U8T8 = 00 + 48).
Using 3 in 386, the pair product of 6 and 6 is 8 + 1 = 9 (U3T3 = 18 + 18).
Adding the three number together we get 0 + 4 + 9 = 13.We attach 3 in 13 as the next digit. But 13 has two digits so we carry over the 1 in 13. So the last digit from the previous step which is 1 becomes 2 (got by carrying over 1+1).
=Â Â U6 T6
=Â Â Â Â Â U8 T8
=Â Â Â Â Â Â Â Â Â U3 T3
0Â Â 0Â Â Â 0Â Â 6Â Â Â 6Â Â Â 7Â Â 1 x 386 = 2 3 _ _ _ _ _Step 2.3
Moving to the next set of digits
Using 6 in 386, the pair product of 0 and 6 is 0 + 3 = 3 (U6T6 = 00 + 36).
Using 8 in 386, the pair product of 6 and 6 is 8 + 4 = 12 (U8T8 = 48 + 48).
Using 3 in 386, the pair product of 6 and 7 is 8 + 2 = 10 (U3T3 = 18 + 21).
Adding the three number together we get 3 + 12 +10 = 25.We attach 5 in 25 as the next digit. But 25 has two digits, so we carry over the 2 in 25. So the last digit from the previous step which is 3 becomes 5 (got by carrying over 3 + 2).
=Â Â Â Â Â U6 T6
=Â Â Â Â Â Â Â Â Â U8 T8
=Â Â Â Â Â Â Â Â Â Â Â Â U3 T3
0Â Â 0Â Â Â 0Â Â 6Â Â Â 6Â Â Â 7Â Â 1 x 386 = 2 5 5 _ _ _ _Step 2.4
Moving to the next set of digits
Using 6 in 386, the pair product of 6 and 6 is 6 + 3 = 9 (U6T6 = 36 + 36).
Using 8 in 386, the pair product of 6 and 7 is 8 + 5 = 13 (U8T8 = 48 + 56).
Using 3 in 386, the pair product of 7 and 1 is 1 + 0 = 1 (U3T3 = 21 + 03).
Adding the three number together we get 9 + 13 +1 = 23.We attach 3 in 23 as the next digit. But 23 has two digits, so we carry over the 2 in 23. So the last digit from the previous step which is 5 becomes 7 (got by carrying over 5 + 2).
=Â Â Â Â Â Â Â Â Â U6 T6
=Â Â Â Â Â Â Â Â Â Â Â Â U8 T8
=Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â U3 T3
0Â Â 0Â Â Â 0Â Â 6Â Â Â 6Â Â Â 7Â Â 1 x 386 = 2 5 7 3 _ _ _Step 2.5
Moving to the next set of digits
Using 6 in 386, the pair product of 6 and 7 is 6 + 4 = 10 (U6T6 = 36 + 42).
Using 8 in 386, the pair product of 7 and 1 is 6 + 0 = 6 (U8T8 = 56 + 08).
Using 3 in 386, the pair product of 1 and _ is 3 + _ = 3 (U3T3 = 03 + –-).
Adding the three number together we get 10 + 6 + 3 = 19.We attach 9 in 19 as the next digit. But 19 has two digits, so we carry over the 1 in 19. So the last digit from the previous step which is 3 becomes 4 (got by carrying over 3 + 1).
=Â Â Â Â Â Â Â Â Â Â Â U6 T6
=Â Â Â Â Â Â Â Â Â Â Â Â Â Â U8 T8
=Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â U3 T3
0  0   0  6   6   7  1      x   386 = 2 5 7 4 9 _ _Step 2.6
Moving to the next set of digits
Using 6 in 386, the pair product of 7 and 1 is 2 + 0 = 2 (U6T6 = 42 + 06).
Using 8 in 386, the pair product of 1 and _ is 8 + _ = 8 (U8T8 = 08 + –-).
Using 3 in 386, the pair product of _ and _ is _ + _ = _ (U3T3 = –– + –-).
Adding the three number together we get 2 + 8 + _ = 10.We attach 0 in 10 as the next digit. But 10 has two digits, so we carry over the 1 in 10. So the last digit from the previous step which is 49 becomes 50 (got by carrying over 49 + 1).
=Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â U6 T6
=Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â U8 T8
=Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â U3 T3
0  0   0  6   6   7  1        x   386 = 2 5 7 5 0 0 _Step 2.7
Moving to the next set of digits
Using 6 in 386, the pair product of 1 and _ is 6 + _ = 6 (U6T6 = 06 + –-).
Using 8 in 386, the pair product of _ and _ is _ + _ = _ (U8T8 = –– + –-).
Using 3 in 386, the pair product of _ and _ is _ + _ = _ (U3T3 = –– + –-).
Adding the three number together we get 6 + _ + _ = 6.6 becomes the last digit of the answer. .
=Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â U6 T6
=Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â U8 T8
=Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â U3 T3
0  0   0  6   6   7  1          x   386 = 2 5 7 5 0 0 6This is your final answer.
Hope this clarifies.
Both DS and DD methods are simple checks to quickly verify your answer without having to do the calculation again.
The only way to be 100% sure that you have the right answer is to do the calculation again.
The only time that the DS method will fail is if the Digit Sum of the incorrect answer matches the Digit Sum of the Right answer.
7 x 6 = 42 (DS is 6).
So 15 x 16 should have DS 6 on both sides.
However if you calculate incorrectly and the incorrect answer also has then digit sum 6 then the DS method will fail.
The probability of that happening is approximately 10% of the time. It is exactly around 11.11% of the time. The remaining 88.88% of the time (approximately 90%) it should correctly identify the mistake.
How do we calculate this probability?
The incorrect answers digit sum can be between 1 to 9. The digit sum will identify the error 8 out of 9 times. But 1 out of 9 times it will match with the correct answer digit sum.
So 8/9 = 88.88% (approximately 90%) the DS method will identify the error. But 1/9 times it will fail.
In the example 15 x 16, the DS method will fail if the incorrect answer has DS 6. But it won’t fail if the answer has DS between 1 to 5 and 7 to 9.
DD method has a similar error rate. However since the methods use different computations usually what fails in one method should be caught in the other method. However there are going to be exceptions where both methods fail.
Both these methods are meant to be simple quick checks and nothing is going to replace doing the calculation again which is the only way I know of to get 100% accuracy.
Hope this clarifies.
Ah I see. Most of this seem to be arithmetic operations which are covered in the material.
Let me know how it goes.
What kind of mental math do you do in school?
The reason I am asking is to know whether the book and the course has everything that you do in school.
Will include additional content based on your feedback.
Wow QianHong.
Where are you from?
Would definitely love to hear more about the things that are tested in eagle flight team club.
Please share your techniques here with the community.
You make the number smaller by dividing by prime numbers first 2, 3, 5, 7 and 11.Â
Mostly it will reduce the number unless its a prime number or a multiple of a prime number greater than 11
506 divided by 2 gives 253.Â
253 cannot be divided by 3,5 or 7. How do I know its not divisible so quickly? I use the divisibility test.
A number is divisible by 2 if the last number is even.Â
A number is divisible by 3 if the sum of the digits is divisible by 3
A number is divisible by 5 if the last digits are 0 or 5.Â
For 7 you need to actually divide to check.Â
So we try 11.Â253 divided by 11 gives 23
23 is also a prime number so we can’t reduce it further.ÂThis is the only way to reduce it.
Â
You can combine the prime factors together if you want to reduce steps.Â
For example, the prime numbers of 506 is 2, 11 and 23.Â
We can combine 2 and 11 to get 22.
So we are left with 22 x 23.ÂHope this helps
Welcome to the community Jim. How far into the book are you right now?
Hi Toni,
Any goal you hope to achieve by inculcating this new habit? Like a specific grade or something?
Hi Toni,
Welcome to the community. Looking forward to seeing your progress updates. Please make sure you complete all the workbooks and post it here.
Abhishek
Hi Toni
What do you spend time learning? Is it school or university work or some new skill?
Abhishek
February 28, 2020 at 2:14 pm in reply to: Multiplying single-digit numbers into double-digit numbers: The DS method #10777Ah very useful indeed. If you notice your steps, you are converting the single digit multiplication problem into another multiplication problem 4 x 6 = 24. I would therefore recommend learning the single digit multiplication table by heart.
The method is the same for negative and positive numbers. There is absolutely no difference.
February 19, 2020 at 3:23 am in reply to: Do you have a facebook group to accompany your book? #10531Currently there is no Facebook group. We only have the community forum here on the website.
The $45 price includes a 30% discount.
The current price on Udemy is $65 – https://www.udemy.com/course/speed-mental-math-tricks/
You can purchase the course at $45 from Ofpad.com – https://ofpad.com/mental-math-sp/
Unfortunately we cannot discount the course any further to be fair to people who paid full price.
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