# NEET Physics Nuclei Questions Solved

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If 20 gm of a radioactive substance due to radioactive decay reduces to 10 gm in 4 minutes, then in what time 80 gm of the same substance will reduce to 10 gm
(a) In 8 minutes              (b) In 12 minutes
(c) In 16 minutes             (d) In 20 minutes

(b) 20 gm substance reduces to 10 gm (i.e. becomes half in 4 min. So ${\mathrm{T}}_{1/2}$ = 4 min . Again

$\mathrm{M}={\mathrm{M}}_{0}{\left(\frac{1}{2}\right)}^{\mathrm{t}/{\mathrm{T}}_{1/2}}$

$⇒10=80{\left(\frac{1}{2}\right)}^{\mathrm{t}/4}⇒\frac{1}{8}={\left(\frac{1}{2}\right)}^{3}={\left(\frac{1}{2}\right)}^{\mathrm{t}/4}⇒$ t = 12 min.

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