The half life of a radio active substance is 10 days. This means that

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The substance completely disintegrates in 20 days.

The substance completely disintegrates in 40 days.

1/8 part of the total number of active nuclei of the substance will be left infact at the end of 40 days.

7/8 th part of the total number of active nuclei of the substance disintegrates in 30 days.

answer is D.

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Detailed Solution

After 30 days (i.e. 3 half lives) number of active nuclei left is given by N=N023T3=N08.So total number of nuclei disintegrates in 30 days =N0−N08=78N0

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