\[6\frac{3}{4} = 6,75 \approx 6,8\]
\[18\frac{7}{8} = 18,875 \approx 18,9\]
\[\mathrm{\Delta}x = |6,8 - 6,75| = |0,05| =\]
\[= 0,05\]
\[\delta x = \frac{\mathrm{\Delta}x}{a} = \frac{0,05}{6,8} = \frac{5}{680} = \frac{1}{136}\]
\[\mathrm{\Delta}x = |18,9 - 18,875| =\]
\[= |0,025| = 0,025\]
\[\delta x = \frac{\mathrm{\Delta}x}{a} = \frac{0,025}{18,9} = \frac{25}{18900} =\]
\[= \frac{1}{756}.\]