By using \(T = 2\pi \sqrt {\frac{l}{g}} \)
\(\Rightarrow \frac{{{T_1}}}{{{T_2}}} = \sqrt {\frac{{{l_1}}}{{{l_2}}}} \)
Hence, \(\frac{{{T_1}}}{{{T_2}}} = \sqrt {\frac{{100}}{{121}}}\)
\(\Rightarrow {T_2} = 1.1\,{T_1}\)
\(\%\) increase = \(\frac{{{T_2} - {T_1}}}{{{T_1}}} \times 100 = 10\,\% \)