The half life of a radioactive substance is 20 minutes. In how much...

Activity of substance is given by, $A = \dfrac{{A}_{0}}{{2}^{n}}$, where $n$ is the number of half-lives.

Now, $\dfrac{{A}_{0}}{16} = \dfrac{{A}_{0}}{{2}^{n}} ⇒ n = 4$

Therefore,

$n = \dfrac{t}{{T}_{\dfrac{1}{2}}} ⇒ t = 4T\dfrac{1}{2} = 4 \times 20min = 80min$