\[\boxed{\text{5.379.}}\]
\[\textbf{а)}\ \frac{18}{15} = \frac{6}{5} = 1\frac{1}{5}\ \]
\[\textbf{б)}\ \frac{21}{14} = \frac{3}{2} = 1\frac{1}{2}\ \]
\[\textbf{в)}\ \frac{55}{33} = \frac{5}{3} = 1\frac{2}{3}\ \]
\[\textbf{г)}\ \frac{168}{40} = \frac{21}{5} = 4\frac{1}{5}\text{\ \ }\]