Tìm x , biết: j) 10/21 − 3x = 3/8 ⋅ 4/15 .

j) \[\frac{{10}}{{21}} - 3x = \frac{3}{8} \cdot \frac{4}{{15}}.\]

   \[\frac{{10}}{{21}} - 3x = \frac{{3 \cdot 4}}{{8 \cdot 15}}\]

\[\frac{{10}}{{21}} - 3x = \frac{1}{{10}}\]

\[3x = \frac{{10}}{{21}} - \frac{1}{{10}}\]

\[3x = \frac{{100}}{{210}} - \frac{{21}}{{210}}\]

\[3x = \frac{{79}}{{210}}\]

\[x = \frac{{79}}{{630}}\]

Vậy \[x = \frac{{79}}{{630}}.\]