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  1. A sample of radioactive element has a mass of 10 gm at an instant t=0. The approximate mass of this element in the sample after two mean lives is
    1. 6.30 gm
    2. 1.35 gm
    3. 2.50 gm
    4. 3.70 gm
Correct Option: B

Using the relation for mean life.

Given : t = 2τ = 2
1
λ

∴ τ =
1
λ

Then from
= 10
1
2= 1.35g
2


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