Ответ:

\(\frac16x+\frac5{12}x=8.4\).

\(\left(\frac2{12}+\frac5{12}\right)x=8.4\), \(\frac7{12}x=8.4\).

\(x=8.4\cdot\frac{12}{7}=14.4\).

Ответ: \(x=14,4\).