c
(c) By using\(N = {N_0}{\left( {\frac{1}{2}} \right)^{t/T}};\)

where \(N = \left( {1 - \frac{7}{8}} \right)\,{N_0} = \frac{1}{8}{N_0}\)

So \(\frac{1}{8}{N_0} = {N_0}{\left( {\frac{1}{2}} \right)^{t/T}}\)

==>\({\left( {\frac{1}{2}} \right)^3} = {\left( {\frac{1}{2}} \right)^{t/5}}\)

==> \(t =15\, days.\)