Question 822995
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Hi,
Find the half-life of a radioactive substance 
if 240 grams of the substance decays to 180 grams in 9 years.
The Idea is to 'personalize' {{{Q(t)=Q[0]e^(-kt) }}} by finding the 'k' value
according to the information given.
180/240 = e^-kt
 .75 = e^-9k
 ln(.75)/-9 =  k  = .0320
 {{{Q(t)=Q[0]e^(-.0320t) }}}
 Find the half-life of this radioactive substance: {{{Q(t)/Q[0] = 1/2}}}
  {{{.5 = e^(-.0320t) }}}
  ln(.5)/-.0320 = t = 21.66 yrs