Atoms And Nuclei

The half-life of a radioactive substance is \(20 \mathrm{~min}\). The approximate time interval \(\left(t_2-t_1\right)\) between the time \(t_2\), when \(\frac{2}{3}\) of it has decayed and time \(t_1\) when \(\frac{1}{3}\) of it had decayed is