Exponential decay

  • #1
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Homework Statement


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


Homework Equations


Q(t) = Qe^(rt)

The Attempt at a Solution


i tried 0.9Q=Qe^(-0.138629t)
0.9=e^(-0.138629t)
ln0.9=lne^(-0.138629t)
ln0.9=-0.138629t
ln0.9/-0.138629 =t
t=0.760018
and i was not able to find the answer
 

Answers and Replies

  • #2
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If 90% has decayed, how much is left?
 
  • #3
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Thank you i got 0.1Q=Qe^(-0.138629t)
t= 16.6
 

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