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Homework Help: Half life and

  1. Jun 1, 2008 #1
    1. The problem statement, all variables and given/known data

    The Half-life of a radioactive substance is 5 hours. A sample is tested and found to contain 0.48g of the substance.
    How much of the substance was present in the sample 20 hours before the sample was tested?

    2. Relevant equations

    3. The attempt at a solution
    i did like this....

    20/5 = 4 half life

    0.48---->0.24--->0.12--->0.06--->0.003g amount of substance was present.

    am i right?
  2. jcsd
  3. Jun 1, 2008 #2


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    Homework Helper

    Right idea, but you want the other direction in time...
  4. Jun 1, 2008 #3
    why the other direction? can you please explain? that's where i am having problem....in calculating.....
  5. Jun 1, 2008 #4


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    Homework Helper

    You are calculating forward in time when you used the half-life. So you are figuring out how much of the radioactive material is left after it was sampled. The question asks you to figure out how much of the material was present before the sample was made...
  6. Jun 1, 2008 #5
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