Tìm giá trị của x , biết: (a) 6/10 = x/− 15 . (b) 3/4 − ( x + − 2/5 ) = − 3/7 + 20/14 .

a) \(\frac{6}{{10}} = \frac{x}{{ - 15}}\)

\(10 \cdot x = 6 \cdot \left( { - 15} \right)\)

\(10x = - 90\)

\(x = - 9\).

Vậy \(x = - 9\).

b) \(\frac{3}{4} - \left( {x + \frac{{ - 2}}{5}} \right) = \frac{{ - 3}}{7} + \frac{{20}}{{14}}\)

\(\frac{3}{4} - x - \frac{{ - 2}}{5} = 1\)

\(x = \frac{3}{4} - \frac{{ - 2}}{5} - 1\)

\(x = \frac{3}{{20}}\).

Vậy \(x = \frac{3}{{20}}\).