SOLUTION: The half-life of iodine-131 is 7.2 days. How long will it take for a sample of this substance to decay to 30% of its original amount? Round to one decimal place.

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Question 852103: The half-life of iodine-131 is 7.2 days. How long will it take for a sample of this substance to decay to 30% of its original amount? Round to one decimal place.
Answer by ewatrrr(24785) About Me  (Show Source):
You can put this solution on YOUR website!
 

Hi,

I%28t%29+=+I%5B0%5D+e%5E%28-kt%29  0r  I%28t%29%2FI%5B0%5D+=++e%5E%28-kt%29 

Finding 'k' first

 .5 = e^(-7.2k)

ln%28.5%29%2F%28-7.2%29 = k = .0963

 .3 = e^(-.0963t)

ln%28.3%29%2F%28-.0963%29 = t