Tìm x biết: (a) x − 4/5 = 2/3 . (b) 2/3 − x = − 5/12 .

a) \(x - \frac{4}{5} = \frac{2}{3}\)

\(\begin{array}{l}x = \frac{2}{3} + \frac{4}{5}\\x = \frac{{10}}{{15}} + \frac{{12}}{{15}}\\x = \frac{{22}}{{15}}\end{array}\)

Vậy \(x = \frac{{22}}{{15}}\).

b) \(\frac{2}{3} - x = \frac{{ - 5}}{{12}}\)

\(\begin{array}{l}x = \frac{2}{3} + \frac{5}{{12}}\\x = \frac{8}{{12}} + \frac{5}{{12}}\\x = \frac{{13}}{{12}}\end{array}\)

Vậy \(x = \frac{{13}}{{12}}\).

c) \(\frac{{5 - x}}{{20}} = \frac{3}{5}\)

\(\begin{array}{l}\frac{{5 - x}}{{20}} = \frac{{12}}{{20}}\\5 - x = 12\\x = 5 - 12\\x = - 7\end{array}\)

Vậy \(x = - 7\).