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Reverse Percentages

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    The question is:
    In a class there are 9 people absent. This is 20% of the class
    How many are there in the class when no-one is off ill?

    so to work out i did: 1.8*9=16.2

    but 10% of the question is 4.5 and 4.5*10= 45 (Half of 20 is 10 and half of 9 is 4.5)

    so i recon 45 is the answer but how do I work it out the first way?
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    (Original post by S_Kid)
    The question is:
    In a class there are 9 people absent. This is 20% of the class
    How many are there in the class when no-one is off ill?

    so to work out i did: 1.8*9=16.2

    but 10% of the question is 4.5 and 4.5*10= 45 (Half of 20 is 10 and half of 9 is 4.5)

    so i recon 45 is the answer but how do I work it out the first way?
    Yes, 45, is the correct answer.

    Lets say that the total number of students in the class are  \displaystyle x

    So we can form the equation as below,
     \displaystyle \frac{20}{100} \times x = 9
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    (Original post by S_Kid)

    so i recon 45 is the answer but how do I work it out the first way?
    What do you mean "the first way"

    Where did 1.8 come from

    9/0.2 = 45
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    (Original post by TenOfThem)
    What do you mean "the first way"

    Where did 1.8 come from

    9/0.2 = 45
     0.2 \times 9 = 1.8
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    9 = 20%
    45 = 100%
  6. Offline

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    (Original post by raheem94)
     0.2 \times 9 = 1.8
    Maybe but ... why would they then do 1.8x9?
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    If 9 equals 20% then 100% will equal 9 multiplied by five. This is 45. The reason why you multiply by 5 is because you want to get from 20% to 100% (the full amount).
  8. Offline

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    (Original post by TenOfThem)
    Maybe but ... why would they then do 1.8x9?
    There is no reason to do this. I was just telling you that this was probably the way in which the OP worked out 1.8, however, it looks wrong to do this.
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    (Original post by raheem94)
    There is no reason to do this. I was just telling you that this was probably the way in which the OP worked out 1.8, however, it looks wrong to do this.
    Yeah .. the method was clearly wrong I was just trying to figure out what their thinking was
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    Seriously, if you can't do this, go to primary school.
  11. Offline

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    (Original post by AGrumpyMole)
    Seriously, if you can't do this, go to primary school.
    The OP is asking for help, and you aren't being of any help by giving such harsh opinions.

    There is no shame on asking a simple question, he is at least trying to learn it.
  12. Offline

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    (Original post by AGrumpyMole)
    Seriously, if you can't do this, go to primary school.
    It is only easy if you know the answer.

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