DD Method vs DS Method

Home Community Questions & Answers Mental Math Q & A DD Method vs DS Method

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  • #7365
    srirampknss
    Participant
    • Topics: 1
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    Hey guys, I am Sriram. I love working around numbers.

    I started Mental Math book and I have a small question. I feel DS method is faster and easier than DD method. But how do you know when DS method isn’t correct and when to use DD method.

    #7370
    Ofpad
    Participant
    • Topics: 0
    • Replies: 158

    Hi Sriram,

    Welcome to the community.

    The DS Method is used when you need to check your answer quickly. You will be able to check the answer with 90% accuracy if you use the DS method.

    The only way to know if the DS method is not correct is to double check your answer with the DD method.

    Hope this helps.

    #8365
    Areadien
    Participant
    • Topics: 4
    • Replies: 7

    The DD method will catch decimal point placement and number transposition errors. For example, as the videos say, if you write 16.1 / 7 = 23, then the DD method will produce 1 instead of 10, while the DS method won’t catch it. Then, if you write 242 instead of 224, the DS method will provide the same answer for each, while the DD method won’t.

    #8367
    Areadien
    Participant
    • Topics: 4
    • Replies: 7

    But I do have a question myself. Where would the DS method catch something that the DD method wouldn’t? The lecture videos didn’t say in that section.

    #8371
    Abhishek R
    Keymaster
    • Topics: 0
    • Replies: 39

    Hi Areadien,

    As far as I know, there is no scenario where the DD method will fail where the DS method succeeds.

    The DD method is more accurate than DS Method. You can be sure 90% of the time with the DS method. However, with the DD method, you will have more than 95% certain.

    We still cover the DS method because it is a lot faster to do than the DD method.

    #8505
    Areadien
    Participant
    • Topics: 4
    • Replies: 7

    Actually, I came up with one. I multiplied 35 * 67 and got 2455 instead of 2345, and I knew it was wrong only because of the DS method.

    So the DS method will fail where the DD method succeeds when the correct answer is added to a non-zero multiple of 11 that is not a also a multiple of 9. So coming up with 3335 would have made the DS and DD methods both succeed, but the answer would have been wrong.

    #8512
    Abhishek R
    Keymaster
    • Topics: 0
    • Replies: 39

    Hi Areadien,

    Yes, you are right. Both methods are not 100%. They are meant to check your answer with a reasonable level of accuracy but there will always be cases where they don’t catch the error.

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