It will take 28 days to let the radioactivity fall below 25%.
Half life can be described as the time required by the radioactive compound to reduce to half of the original value of concentration.
It is given that
Radioactive phosphorus-32 decays with a half-life of 14.0 days
25% of the initial measure at what time =?
For 50% decay the time needed is 14 days
for decay of the next 50% of the amount left the time taken will be another 14 days
2nd half life = 25% decay of the substance = 28 days
Therefore it will take 28 days to let the radioactivity fall below 25%.
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