Left fraction of activity \(\frac{1}{16}\)
\(\because \frac{ R }{ R _0} =\left(\frac{1}{2}\right)^{ t / T }\)
\(\frac{1}{16} =\left(\frac{1}{2}\right)^{ t / 20}\)
\(\left(\frac{1}{2}\right)^4 =\left(\frac{1}{2}\right)^{ t / 20}\)
\(4 =\frac{ t }{20}\)
\(t =80\,min\)