Tìm x: 

- Hướng dẫn giải

$$\begin{gathered} \frac{11}{12}-\left(\frac{2}{5}+x\right)=\frac{2}{3} \\ \begin{array}{l}\frac{2}{5}+x=\frac{11}{12}-\frac{2}{3}\\ \frac{2}{5}+x=\frac{11}{12}-\frac{8}{12}\\ \frac{2}{5}+x=\frac{1}{4}\end{array} \end{gathered}$$

$\begin{array}{l}x=\frac{1}{4}-\frac{2}{5}\\ x=\frac{-3}{20}\end{array}$