Week 4 Video Lessons

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    • #28838

      Please use this thread to post your questions/Comments/impressions/Learnings etc on topics covered in week 4 video lessons

    • #30288
      Maurice Perry

      I never thought Astronomy would be interesting. Then came the video on how the 9 point circle is clearly related to the subject. Im intrigued!

    • #30334
      Preeti Pathak


      In lesson 14, multiplying near base 100(other bases also),the vertcally and crosswise method is really a faster way to get the answer as compared to the conventional method.And the added advantage is that it works for other bases (ex,200,200 etc) also.

      The three proofs(Arithmetic,Geometric and Algebraic)are well explained to answer why the method(for multiplying numbers near base) works.

      Extension of base multiplication method ,use of “Proportionately” in solving other multiplication problems is helpful.Also,it saves lot of time in doing the calculations.

      Table patten,Sequence Pattern are intresting topics.Really enjoyed representing the sequence on the 9-point circle.I drew the table pattern(for 7 times table).

      The use of “reversing” is explained in a simple manner in “Thinking of a number”.

      Also, the use of equations in finding the “unknown number” is explained in a good way, especially when we are doing it mentally .

      Had a great learning.






    • #30348
      Fabio Zanatta


      in lesson “multiplying near base 100”, if we consider the multiplication 89×97=8633

      89 is deficient from 100 of 11

      97 is deficient from 100 of 3

      The first part of the result is 89-3=86 (3 is the deficiency of 97 form 100)

      The second part of the resutl is 11×3=33


      I observed that we can calculate 86 in other ways:

      • if we consider deficiency of both numbers (97 and 89) we have: 100-11-3=86
      • if we consider 97 and the deficiency of 89 we have: 97-11=86

      It’s correct?


      • #30416

        That’s right Fabio. These are all correct ways to get the left-hand side.

    • #30368


      It is very interesting and amazing to learn about various methods to perform multiplication mentally. However, I am stuck at one particular scenario and appreciate help there.

      When there is 2-digit bar number in the right hand side of the result but actual answer should only contain single digit there thus applying carry on to the left side digits is giving wrong answer(I am not sure where/what I am doing wrong). Please see this example.

      46 * 38 –> considering base 40

      4 6 + 6

      38 – 2

      so 44 / (12). Since base was 40, I multiplied the left side digits with 4.

      176 / (12) –> now removing bar number gives 88 in the right side and 175 in the left (as we borrow 1 from 176).

      175 / 88 –> 10’s place 8 will be a carry on to 100’s place ?

      So I got the answer 1838 . However, the correct answer is 1748.

      Please correct my process. Am I messing up at carry ons or bar numbers?

      Thank you

      • #30376

        Hi Satya,

        After getting 176/(12), we should subtract (1) from 176 as it is a carry and that will give us 175/(2). Now we have to remove (2) and the answer will be 1748.

        Hope that helps!!



      • #30443

        Got it…Thank you Mallika.

    • #30377

      As 40 is our base number, it has single 0 hence we will have single digit in the right half of the answer.

      • #30445

        Yeah, that is why I moved one digit to right side but I did that after converting the bar number to positive so it got added.

        Seems we have to do it prior to converting the bar number so it will be a subtraction and one more subtraction to convert the bar number.


    • #30412
      Preeti Pathak


      In lesson 15, for multiplying 204  with 107 ,Iwe used base as 100. Can we do it with base as 200 by first multiplying 107 by 2.?I tried this way.

      base =200    204     +4


      214      +14


      218/56 and if we do half of it (since 107 was doubled)we will get 10928(not 10914).

      But with base as 200,if we multiply by 2(i.e.218*2),we will get 43656 and then again need to half of it giving the answer to be 21828.(however if we still make it half,will get the answer). Please explain if we can do it with base 200 and where I am getting wrong?



      • #30414

        Hi Preeti,

        Because you are using a base of 200 the left-hand part of 218/56 has to be doubled.

        That gives 436/56, and when that is halved we get 21828.

        It looks like you tried to do the final halving too early.

      • #30446
        Preeti Pathak

        Thanks Ken. Got it now where I was doing mistake. we have to do final halving at last.

    • #30422
      Savita P Kulkarni

      Hi Priti, good to see you,

      Thanks Mallika and Satya for clarifying, I was a bit confused for the bar on 12 and moving the 1 with a bar to left side.

      I loved the multiplication table of 4 with a pattern explained with a 9 point circle.

      I am done with all the lessons and the quiz, but it is not allowing me to mark complete.

      Thank you,

    • #30441

      Hi Ken, I have a doubt regarding quiz. Will I be able to do week 4 quiz next week or should we complete it by today?


      • #30461

        Hi Mallika,

        This week’s test should completed by the end of today.

    • #30447
      Maurice Perry


      In Lesson 15, im stuck when it comes to 309×104.309 havled doesn’t work (154.5). 300 is the closest base. What am I missing?

      • This reply was modified 8 months, 2 weeks ago by Maurice Perry.
      • #30459
        Preeti Pathak

        Hi Maurice !

        I tried this multiplication with base as 100. I would share.


        309  * 104

        if we take one-third of 309, we will get 103



        103         +3 (deficiency from 100)


        104          +4(deficiency from 104)


        (103+4)/ 12=107/12

        and now, we will have to multiply by 3(since in the beginning,we took 1/3 of 309.

        s0 3*(10712)=    32136 is the answer





      • #30498
        Anupama Cherukuri

        Hi Maurice 309 halve  154.5 also works

        Base 100

        154.5    + 54.5

        104       + 4


        158.5     + 218

        Consider 158.5   as 15850 then add 218  it is 16068 then double it 32,136.

    • #30460
      Shikha Dhingra

      Hi Maurice

      Solution to  309 x 104  can be like this:

      first multiply by  3   you will get 309 x 312

      now taking 300 as base solve 309 x 312

      321 / 108

      now multiply by 3 as 300 is the base

      You will get        963/108   =   96408

      now you have to divide by 3 as you multiply by 3 in the starting to make the base 300

      your answer will be  96408  divide by 3 is     32136

      I  hope it will help

    • #30495
      Maurice Perry

      Thank you Preeti and Shika!! Definitely was over thinking this matter.

    • #30835
      Priyanka Dugar

      Hi Ken,

      I am not able to mark lesson 16 complete.

      • #30996

        Hi Priyanka,

        Sorry to hear that. Please submit a ticket.


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