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A radioactive isotope has a half life of...

A radioactive isotope has a half life of 5 yrs. How long will it take the activity to reduce to `3.125%`?

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The correct Answer is:
`25 yr`
Here, `t=?, T=5` years, `N/(N_(0))=3.125/100`
As `N/(N_(0))=(1/2)^(n)=(1/2)^(t//T) :. 3.125/100=1/32=(1/2)^(5)`
`t/T=5, t=5T=5xx5 years=25yr`
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