Incorrect. First rewrite each term using $\displaystyle{30}{x}{y}\usepackage{cmbright}\Huge\$ as the denominator:
$\displaystyle\frac{{1}}{{{10}{x}}}-\frac{{2}}{{{15}{y}}}+\frac{{1}}{{{30}{x}{y}}}=\frac{{{1}}}{{{10}{x}}}\cdot\frac{{{3}{y}}}{{{3}{y}}}-\frac{{2}}{{{15}{y}}}\cdot\frac{{{2}{x}}}{{{2}{x}}}-\frac{{1}}{{{30}{x}{y}}}\usepackage{cmbright}\Huge\$

Simplify each and then combine the numerators.