$\"\"$

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The exponential decay function is $\"\"$ .

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The half-life of iodine 131 is $\"\"$ days.

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Hence, time period is $\"\"$ and $\"\"$

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Consider the initial amount is $\"\"$ .

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Substitue all the values.

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$\"\"$

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$\"\"$

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Apply natural logarithm on each side.

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$\"\"$

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$\"\"$

\

$\"\"$

\

$\"\"$

\

$\"\"$

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There,the decay constant is $\"\"$ .

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$\"\"$

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Find the time ,when 10% of the iodine131 remains and the farmers need to wait to use this hay.

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$\"\"$ and $\"\"$ and $\"\"$

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Susbtitute all the values in $\"\"$ .

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$\"\"$

\

$\"\"$

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Take natural logarithm on each side.

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$\"\"$

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$\"\"$

\

$\"\"$

\

$\"\"$

\

$\"\"$

\

Therefore, $\"\"$ .

\

Therefore, the farmers need to wait to use this hay for $\"\"$ days.

\

$\"\"$

\

The farmers need to wait to use this hay for $\"\"$ days.