\[\boxed{\mathbf{П.\ 18}}\]
\[\textbf{а)}\ 0,51 = \frac{51}{100} = \frac{357}{700};\ \ \]
\[\ \frac{17}{35} = \frac{340}{700}\]
\[0,51 > \frac{17}{35}\]
\[\textbf{б)}\ 0,56 = \frac{56}{100} = \frac{14}{25} = \frac{294}{525};\ \ \]
\[\ \frac{11}{21} = \frac{275}{525}\]
\[\frac{11}{21} < 0,56\]
\[\textbf{в)}\ \frac{13}{16} = \frac{169}{208};\ \ \ \frac{11}{13} = \frac{176}{208}\]
\[\frac{13}{16} < \frac{11}{13}\]
\[\textbf{г)}\ \frac{13}{15} = \frac{247}{285};\ \ \frac{15}{19} = \frac{225}{285}\]
\[\frac{13}{15} > \frac{15}{19}\]