Hi Dennis,

You can safely skip the basic multiplication module, as long as you mastered the LR method. The Basic Multiplication Module is there more for completeness of the course, which is why I cover it after the LR method. Many students enroll in the course looking for specific techniques for specific situations. The Basic Multiplication module is tailored for these students who would otherwise find the mental math course incomplete.

That said, the same does not apply to any other lesson in the program. The Bridge, Vitruvian Man Method and UT Method for multiplication are all different ways to multiply that will work faster than the LR method for complex numbers like 3 Digit x 3 Digit multiplication or 5 Digit x 2 Digit Multiplication. You should spend time mastering those lessons and should not solely rely on LR method for this reason.

Let me know if this clarifies.

Abhishek

Hi Dvora,

The methods in the video lectures can easily be extended to 4 or 7 digit numbers.

Bridge & Vitruvian Man Method

You can use the bridge or the vitruvian man method for this but it is a little more complicated than 3 digit numbers. It can still be done entirely in your head if you develop a good working memory as you calculate. Only difficult part would be remembering the numbers you already calculated without writing anything down. I will show you an example.

2567 x 5869

Start with first set of numbers 2567 x 5869

2 x 5 = 10

Your answer starts with 10_ _ _ _ _ _

Next you multiply the first set of outer pairs and add them together 2567 x 5869

(2 x 8) + (5 x 5)  = 16 + 25 = 41

Next digits of your answer 141_ _ _ _ _ _

Next set of outer pairs is 2567 x 5869

(2 x 6) + (5 x 8) + (6 x 5) = 12 + 40 + 30 = 82

Next digits of your answer 1492 _ _ _ _ _

Next set of outer pairs is 2567 x 5869

(2 x 9) + (5 x 6) + (6 x 8) + (7 x 5) = 18 + 30 + 48 + 35= 131

Next digits of your answer 15051 _ _ _ _

Next set of outer pairs is 2567 x 5869

(5 x 9) + (6 x 6) + (7 x 8) = 45 + 36 + 56 = 137

Next digits of your answer 150647 _ _ _

Next set of outer pairs is 2567 x 5869

(6 x 9) + (7 x 6) = 54 + 42 = 96

Next digits of your answer 1506566 _

Last set of numbers  2567 x 5869

7 x 9 = 63

Next digits of your answer 15065723

And your final answer is 15,065,723

LR Multiplication

You will also use the LR multiplication by rounding up the numbers. Like the following

2567 x 5869

5869 can be rounded up by 131 to get 6000.

2567 x (6000 – 131) = 2567 * 6000 – 2567 * 131 = 15,402,000 – 336,277 = 15,065,723.

Alternatively if you are comfortable doing 2 digit multiplication instead then you can round up to 5900 instead of 6000 like the following

2567 x (5900 – 31) = 2567 * 5900 – 2567 * 31 = 15,145,300 – 79,577 = 15,065,723.

Squaring

You also asked for squaring 6422×6422. It can also be done for higher digits.

Rounding down by 22

6444 x 6400 + (22 x 22) = 6444 x 6400 + 484 = 41,242,084

Hope this clarifies.

Hi Harsh,

Thank you for asking this question and I am not sure how I missed adding this in the lecture. When the subtraction of the DS is negative you add 9 to the negative number.

21 – 14 = 7

3 – 5 = -2 (+9) = 7.

Another example

24 – 7 = 17

6 – 7 = -1 (+9) = 8 (Which is the digit sum of 17 = 1 + 7 = 8)

For your particular problem

9128 – 5595 = 3533

2 – 6 = -4 (+9) = 5 (Which is the digit sum of 3533 = 5)

Thank you for asking this question. Will update the lecture so its more clear. 🙂

Hope this clarifies.

Hi Max,

When you are doing the math problem, you might lose track of which numbers you have to calculate. So you place you forefinger and your middle finger on top of the two numbers. The number on which your forefinger is place should be the U and the number on which the middle finger is place should be the T. When you calculate the next digits, you just move your fingers across the paper/computer screen.

Hope this clarifies.

When the numerator is greater than the denominator you don’t have to apply any trick. You can just see the numbers and come up with your answer immediately. Its just a matter of how you write the numbers.

21/20 can be written as (20 + 1)/20 which can be written as 20/20 + 1/20 which can be written as 1 + 1/20

Similarly you can write 1013/1012 as 1 + 1/1012

Equating the two fractions

1 + 1/20 = 1 + 1/1012

1/20 = 1/1012

Cross multiplying we get 1012 > 20

So in summary when the numerator is greater than the denominator, all you have to do is subtract (numerator – denominator) for both fractions and cross multiply. 

(21 – 20)/20 = (1013 – 1012)/1012

1/20 = 1/1012

Cross multiplying we get 1012 > 20

This of course cannot be done when the numerator is smaller than the denominator.

Thank you for the support Imran 🙂

Hi Imran,

The method taught in school can be found here – http://www.quickanddirtytips.com/education/math/how-to-compare-fractions

To find approximately which fraction is higher, there are 3 steps.

Step 1 – Find the amount you must multiply to the numerator to bring it close to the denominator.

In the example 127/255 Vs 162/320 we can see the amount is approximately 2 for both fractions.

If the numbers were different both fractions, then you can directly go to step 3 and say that the smaller number is greater fraction. But here in this example we have to do additional step of step 2 because we multiplied both fractions with the same number 2 to bring it close to the denominator.

Step 2 – Find whether you must add or subtract something to bring the answer of the multiplication exactly close to the denominator

127 * 2 = 254. This number 254 is lesser than the denominator 255 by 1.

So to get exactly 255 you must multiply more than 2. You don’t have to find out exactly how much this value should be, but for this explanation I will say the exact number is 2 + x. (Where x is some decimal number greater than 0 but less than 1)

Similarly 162 * 2 = 324. This number is greater than the denominator 320 by 4.

To get exactly 320, you must multiply less than 2. So it is 2 – y.  (Where y is a decimal number greater than 0 but less than 1)

Step 3 – The greater fraction will have the smaller number. 

In this case (2 – y) is the smaller number and so 162/320 is the greater fraction.

If the numbers you multiplied the numerator with was different in Step 1, (eg 2 for fraction A and 3 for fraction B) , then you can skip Step 2 and immediately find the answer.

This is one way I would guesstimate which number is greater quickly.

Hope this explanation helped.