\[\left| \frac{5}{6} - 0,84 \right| = \left| \frac{5}{6} - \frac{21}{25} \right| = \left| \frac{125}{150} - \frac{126}{150} \right| =\]
\[= \left| - \frac{1}{150} \right| = \frac{1}{150}\]
\[Ответ:\ \frac{1}{150}.\]