Explanation :
$ \mathrm\delta=\frac{\mathrm{PL}^3}{3\mathrm{EI}} $
For square section
$ \mathrm I=\frac{\mathrm a^4}{12} $
Hence, $ \mathrm\delta\propto\frac1{\mathrm a^4} $
$ \begin{array}{l}{\mathrm\delta}_1\propto\frac1{\mathrm a^4}\\{\mathrm\delta}_1\propto\frac1{\left(1.19\right)^4\mathrm a^4}\end{array} $
% decrease in tip defection
$ =\left(\frac{{\mathrm\delta}_2}{{\mathrm\delta}_1}-1\right)\times100=\left[\left(\frac1{1.19}\right)^4-1\right]\times100=50.133\% $