b
(b) For first pipe \({n_1} = \frac{v}{{4{l_1}}}\)

and for second pipe \({n_2} = \frac{v}{{4{l_2}}}\)

So, number of beats = \({n_2} - {n_1} = 4\)

\( \Rightarrow \)\(4 = \frac{v}{4}\left( {\frac{1}{{{l_2}}} - \frac{1}{{{l_1}}}} \right)\)

\( \Rightarrow \)\(16 = 300\left( {\frac{1}{{{l_2}}} - \frac{1}{1}} \right)\)

\( \Rightarrow \)\({l_2} = 94.9\)\(cm\)