a
(a) Number of half lives in two days four substance \(1\) and \(2\) respectively are \(n_1 =\frac{{2 \times 24}}{{12}} = 4\) and \({n_2} = \frac{{2 \times 24}}{{1.6}} = 3\)

By using \(N = {N_0}{\left( {\frac{1}{2}} \right)^n}\)

==> \(\frac{{{N_1}}}{{{N_2}}} = \frac{{{{({N_0})}_1}}}{{{{({N_0})}_2}}} \times \frac{{{{\left( {\frac{1}{2}} \right)}^{{n_1}}}}}{{{{\left( {\frac{1}{2}} \right)}^{{n_2}}}}}\)

\( = \frac{2}{1} \times \frac{{{{\left( {\frac{1}{2}} \right)}^4}}}{{{{\left( {\frac{1}{2}} \right)}^3}}} = \frac{1}{1}\)