Question

The half life of a radioactive substance is 20 minutes. The approximate time interval $\left(t_{2}-t_{1}\right)$ between the time $t_{2}$ when $\frac{2}{3}$ of its has decayed and time $t_{1}$ when $\frac{1}{3}$ of its had decayed is

• A. $28 min$
• B. $7 min$
• C. $14 min$
• D. $20 min$

Answer 1

Answer: (D)

At $ t=t_{2}, N_{2}=\frac{N}{3}$ (Left)
$t=t_{1}, N_{1}=\frac{2 N}{3} $
$ \Rightarrow t_{2}-t_{1}=$ half life $=20 min$