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

• A. 15
• B. 30
• C. 45
• D. 60

Answer 1

Answer: (D)

60

Answer 2

$N_1 = 0.6 \, N_0$ $N_2 = 0.15 \, N_0$ $\frac{N_2}{N_1} = (\frac{1}{2} )^2$ so two half life period has passed So time taken $2t_{1/2} = 2 \times 30 = 60$ minutes