Question

The radioactivity of a certain radioactive element drops to 1/64 of its initial value in 30 seconds. Its half life is.

• A. 2 seconds
• B. 4 seconds
• C. 5 seconds
• D. 6 seconds

Answer 1

Answer: (C)

5 seconds

Answer 2

$N = N_{0}\left( \frac{1}{2} \right)^{t/T} \Rightarrow \frac{N_{0}}{64} = N_{0}\left( \frac{1}{2} \right)^{30/T} \Rightarrow T = \frac{30}{6} = 5\sec$