R is original radius
\(\frac {r+10r}{100}\) or \(\frac {11 r}{10}\) is new radius
\(\therefore\) Increment percentage = \(\frac {\pi (\frac {11r}{10})^2 - \pi r^2}{\pi r^2} \times 100\)
= \(\frac {\pi r^2 (\frac {121}{100}-1)}{\pi r^2} \times 100\)
= \(\frac {21}{100} \times 100\%\)
= 21%