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Homework Help: Simple Bacteria doubling problem

  1. Sep 23, 2008 #1
    1. The problem statement, all variables and given/known data

    The doubling period of a baterial population is 20 minutes. At time t = 120 minutes, the baterial population was 80000. With t representing minutes, the formula for the population is p(t)=A e^{kt}.

    k=?
    The initial population at time t = 0 is:?
    The size of the baterial population after 3 hours is: ?



    I attempted this problem and came up with k being (ln(2))/140. The online assignment keeps saying that is the wrong answer. If i can't find k, I can't get anything else. Please help me out.
     
  2. jcsd
  3. Sep 23, 2008 #2

    Dick

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    If the doubling time is twenty minutes then the population at time t=20 is twice the population at t=0. So 2*A*e^(k*0)=A*e^(k*20). Can you solve that for k? Notice the A cancels.
     
  4. Sep 23, 2008 #3
    I am not sure. I can't solve that for k. I don't have A.
     
  5. Sep 23, 2008 #4

    Dick

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    A cancels!
     
  6. Sep 23, 2008 #5
    oh wow silly me. sorry. it does cancel
     
  7. Sep 23, 2008 #6
    i got (ln(2))/20. Now how do i find t=0 if i dont know what A is.
     
  8. Sep 23, 2008 #7

    Dick

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    Good start! You know 80000=A*e^(k*120). Can't you find A from that?
     
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