A radioactive sample had an initial activity of 56 DPM ( disintegration per min ). After 69.3 min, it was found to have an activity of 28 DPM. Find the number of atoms in a sample having an activity of 10 DPM.

A

693

B

1000

C

100

D

10,000

Hard

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Solution

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Correct option is B)

The expression for the activity is $A=λN=t_{1/2}0.693×N $

$t_{21}=69.3min$

Substitute values in the above expression: $10=69.30.693×N $