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At time t=0 , number of nuclei of a radi...

At time `t=0` , number of nuclei of a radioactive substance are 100. At `t=1` s these numbers become 90. Find the number of nuclei at `t=2 s`.

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The correct Answer is:
A
In 1 second, 90% of the nuclei have remained undecayed, so in another 1 second 90% of 90, i.e. 81 nuclei will remain undecayed.
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