\(T_1=\dfrac{t}{20}\left(s\right);T_2=\dfrac{t}{30}\left(s\right)\)
\(\dfrac{T_1}{T_2}=\sqrt{\dfrac{l_1}{l_2}}\Leftrightarrow\dfrac{l_1}{l_2}=\left(\dfrac{T_1}{T_2}\right)^2=\dfrac{9}{4}\Leftrightarrow l_1-\dfrac{9}{4}l_2=0\)
\(l_1-l_2=5cm\Rightarrow\left\{{}\begin{matrix}l_1=9cm\\l_2=4cm\end{matrix}\right.\)