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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 undecayed nuclei at $t = 2 s$

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$63$

$72$

$81$

$90$

answer is C.

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Detailed Solution

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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