DD: The number 10

Home Community Questions & Answers Mental Math Q & A DD: The number 10

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  • #8366
    Areadien
    Participant
    • Topics: 4
    • Replies: 7

    So the videos say to make sure that you have a one-digit number for the DD method, but 10 always produces 10 with the DD method. Since 0 – 1 = -1 and -1 + 11 = 10, I don’t know what to do with this number, if anything. Is 10 a valid number to have, or do I reduce it to 0 as I can with 9 in the DS method?

    #8372
    Abhishek R
    Keymaster
    • Topics: 0
    • Replies: 39

    Hi Adreadien,

    In this case you will retain the -1 as -1 instead of adding 11 to make it positive. Here is how that will turn out:

    Addition
    Check: 14 + 10 = 24
    DD: 3 – 1 = 2

    Subtraction
    Check: 14 – 10 = 4
    DD: 3 – -1 = 4
    DD: 3 + 1 = 4

    Multiplication
    Check: 14 * 10 = 140
    DD: 3 * -1 = (0+1) – 4
    DD: -3 = -3

    Division
    Check: 14 / 10 = 1.4
    DD: 3 / -1 = 1 – 4
    DD: 3 / -1 = -3
    DD: 3 = -3 * -1

    I will add this scenario to the content during the next update.

    Let me know if this answer clarifies your question.

    #8506
    Areadien
    Participant
    • Topics: 4
    • Replies: 7

    This makes sense because 10 * 10 = 100, and the DD of 100 = 1 = -1 * -1.

    Still, sometimes I have to add 11 to the final answer, such as when I multiply 21 by 58. The DD method would show -3, which is the same as 8, which is the same as the DD method for the answer, 1218.

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