b
(b) ${l_1} + {l_2} + {l_3} = 110\,cm$ and ${n_1}{l_1} = {n_2}{l_2} = {n_3}{l_3}$
${n_1}:{n_2}:{n_3}::1:2:3$
$\because \frac{{{n_1}}}{{{n_2}}} = \frac{1}{2} = \frac{{{l_2}}}{{{l_1}}} \Rightarrow {l_2} = \frac{{{l_1}}}{2}$and $\frac{{{n_1}}}{{{n_3}}} = \frac{1}{3} = \frac{{{l_3}}}{{{l_1}}} \Rightarrow {l_3} = \frac{{{l_1}}}{3}$
$\therefore {l_1} + \frac{{{l_1}}}{2} + \frac{{{l_1}}}{3} = 110$ so ${l_1} = 60cm,{l_2} = 30cm,{l_3} = 20cm$