NEET Physics Nuclei Questions Solved

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If 10% of a radioactive material decays in 5 days, then the amount of original material left after 20 days is approximately
(a) 60%              (b) 65%
(c) 70%              (d) 75%

(b) N=N0e-λt

0.9 N0=N0e-λ×55λ=loge10.9          ....(i) 

and xN0=N0e-λ×2020λ=loge1x            ....(ii)

Dividing (i) and (ii) , we get

14=loge(1/0.9)loge(1/x)=log10(1/0.9)log10(1/x)=log100.9log10xlog10x=4log100.9x=0.658=65.8%

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