Make sure you rewrite each fraction as an equivalent fraction. Do not change only the denominator of $\displaystyle\frac{{4}}{{5}}\usepackage{cmbright}\Huge\$ when rewriting it as a fraction with a denominator of 10.

$\displaystyle\frac{{3}}{{10}}+\frac{{4}}{{5}}=\frac{{3}}{{10}}+\frac{{8}}{{10}}=\frac{{11}}{{10}}={1}\frac{{1}}{{10}}\usepackage{cmbright}\Huge\$

The correct answer is $\displaystyle{1}\frac{{1}}{{10}}\usepackage{cmbright}\Huge\$ .