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Q.

If 10% of a radioactive material decays in 5 days, then the amount of the original material left after 20 days is approximately

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a

60%

b

65%

c

70%

d

75%

answer is B.

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Detailed Solution

N=N0e−λt⇒0.9=e−λ5...........(1) After 20 days,N=N0e−λ20⇒NN0=(e−λ5)4⇒NN0=(0.9)4=0.6561⇒NN0×100=65.61%
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If 10% of a radioactive material decays in 5 days, then the amount of the original material left after 20 days is approximately