It will take 24 days for the radiation level of barium-131 to drop to 1/4 of its original level.
We know that radioactive reactions are 1st order reactions.
Given half life of Barium-131 is approximately 12 days.
So to drop to 1/4 of it's original level it will take more 12 days i.e. one more half life.
Total days are : 12+12=24 days
So it will take 24 days for the radiation level of barium-131 to drop to 1/4 of its original level.
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