The half-life of a radioactive substance is 40 years. How long will it take to reduce to one fourth of its original amount and what is the value of decay constant
(a) 40 year, 0.9173/year            (b) 90 year, 9.017/year
(c) 80 year, 0.0173 year            (d) None of these

(c) To reduce one fourth it takes time $\mathrm{t}=2\left({\mathrm{T}}_{1/2}\right)$ = 2$×$40
= 80 years.
Decay constant

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