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Homework Help: Exponential decay

  1. Dec 9, 2014 #1
    1. The problem statement, all variables and given/known data
    superman has been disabled by a nearby amount of kryptonite, which decays exponentially. If Superman cannot regain his power until 90% og the kryptonite disintegrates, then how long will it be before he regain his powers?
    Use r=-0.138629. Round to the nearest day.
    a. 4days c 11days
    b 6days d 17days e 21days

    2. Relevant equations
    Q(t) = Qe^(rt)

    3. The attempt at a solution
    i tried 0.9Q=Qe^(-0.138629t)
    ln0.9/-0.138629 =t
    and i was not able to find the answer
  2. jcsd
  3. Dec 9, 2014 #2


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    If 90% has decayed, how much is left?
  4. Dec 9, 2014 #3
    Thank you i got 0.1Q=Qe^(-0.138629t)
    t= 16.6
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