Phép tính sai là:

\(\frac{9}{{10}} + \frac{1}{2} = \frac{7}{5}\)

\(\frac{{17}}{{15}} - \frac{2}{5} = \frac{{11}}{{15}}\)

\(\frac{{19}}{{30}} - \frac{1}{2} = \frac{4}{{15}}\)

$\frac{11}{20}+\frac{3}{4}=\frac{13}{10}$