First slide

Radio activity

Question

The half-life of a radioactive substance is 30 minutes. The time (in minutes) taken between 40% decay and 85% decay of the same radioactive substance is

Easy

A

15
B

30
C

45
D

60

Solution

$\begin{array}{l} N_{0} = Nuclei at time t = 0 \\ N_{1} = Remaining nuclei after 40 % decay \\ = (1 - 0.4) N_{0} = 0.6 N_{0} \\ N_{2} = Remaining nuclei after 85 % decay \end{array}$
Hence, two half life is required between 40 % decay and 85 % decay of a radioactive substance.
$∴ Time taken = 2 τ_{1 / 2} = 2 \times 30 \min = 60 \min$

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