Question 1140194
x = 0 for year 1990


{{{y=pe^(-kx)}}}
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{{{1/2=e^(-k*5)}}}
{{{ln(1/2)=-5k}}}
{{{k=ln(2)/5}}}
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{{{y=pe^(-.1386x)}}}


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If it is safe to feed the hay to cows when 
15​% of the radioactive isotope​ remains, how long did the farmers need to wait to use this​ hay? 
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Solve for x with  
{{{0.15=e^(-0.1386x)}}}
.
.