The curve between the activity $A$ of a radioactive sample and the number of active atoms $N$ is

The decay constant of the end product of a radioactive series is

Half-life of a radioactive substance is $20\,minute$ . The time between $20\%$ and $80\%$ decay will be ......... $min$

Assertion : If the half life of a radioactive substance is $40\, days$ then $25\%$ substance decay in $20\, days$

Reason : $N = {N_0}\,{\left( {\frac{1}{2}} \right)^n}$

where, $n = \frac{{{\rm{time\, elapsed}}}}{{{\rm{half \,life \,period}}}}$

Draw a graph of the time $t$ versus the number of undecay nucleus in a radioactive sample and write its characteristics. 

The half-life of a radioactive substance is $T$. The time taken, for disintegrating $\frac{7}{8}$ th part of its original mass will be