The half life of a radioactive substance is 20 minutes. The aproximate time interval $(t_2 - t_1)$ between the time $t_2$ when $\frac{2}{3}$ of it has decayed and time $t_1$ and $\frac{1}{3}$ of it had decayed is :

Question

( A ) 20 min
( B ) 14 min
( C ) 28 min
( D ) 7 min