Ответ:
\[\frac{5}{14}x-\frac{4}{21}x=12-7.5\]
\[\left(\frac{15}{42}-\frac{8}{42}\right)x=4.5\]
\[\frac{1}{6}x=4.5,\qquad x=27.\]
Ответ: 27.
\[\frac{5}{14}x-\frac{4}{21}x=12-7.5\]
\[\left(\frac{15}{42}-\frac{8}{42}\right)x=4.5\]
\[\frac{1}{6}x=4.5,\qquad x=27.\]
Ответ: 27.