Friday 30 September 2016

999 - emergency maths

Is it always, sometimes or never true that when you multiply a number by 9, the sum of it's digits is also 9?


Prove your answer with as many examples as you can!

13 comments:

  1. That is false 9 times 9 is 81 that shows that not true.

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  2. Its maybe true because 9x11=99 or maybe not because only 9x11=99 and the other ones are other numbers for example 3x9=27 and you could do 1x9=9 instead anything else like 4x9=36 so Ithink is sometimes.

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    Replies
    1. Still thinking about the number 9, can anyone tell me what the notice about the sum of the digits in the answer to 9 x 11?

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  3. its never true that when you multiply a number by 9,the sum of its digits is also 9.

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  4. 9x1=9
    9X2=18
    9x3=27
    9x4=36
    9x5=45
    9x6=54
    9x7=63
    9x8=72
    9x9=81
    9x10=90
    9x11=99
    9x12=108

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    Replies
    1. It is sometimes true because if you times 9 and the other number ends in 1 it will have 9 as a digit

      For example
      9x1=9 or9x11=99 9+9 or 9x2 is 18 which is not 9 2x9=18+1=19

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    2. In how many of your answers Joel, does the sum of the digits equal 9?

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    3. yes joel because all you have done is the nine times tables can you find a two diget number that ends in nine and if you can set your self a challenge try a three diget number like

      909x1=909

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    4. The combinations you made Joel you can make to 9 time tables is 2 answers that end with 9

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  5. look carefully at the nie times table and youll see the numbers do add up to nine look

    2x9=18
    but look 18 seperate the numbers 1 8 and add them 1+8=9

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  6. sometimes when you times by nine it has a nine.

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  7. 9+9+9+=26
    10+10+10=30
    20+20+20=60
    30+30+30=90

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