$a) \dfrac{2}{3} : x - 0,5 = 1\dfrac{1}{2}$
$\dfrac{2}{3} : x - \dfrac{1}{2} = \dfrac{3}{2}$
$\dfrac{2}{3} : x = 2$
$x = \dfrac{2}{3} :2$
$x = \dfrac{1}{3}$
Vậy `x=1/3`
$b) 1 - (13\dfrac{5}{6} - x + 5\dfrac{1}{2}) : 15\dfrac{1}{2} = 0$
$1 - (13\dfrac{5}{6} - x + 5\dfrac{1}{2}) = 0$
$\dfrac{83}{6} - x + \dfrac{11}{2} = 1$
$\dfrac{53}{6} - x = 1$
$x = \dfrac{53}{6} - 1$
$x = \dfrac{47}{6}$
Vậy $x=\dfrac{47}{6}$
$c) (x + 20 ) + ( x - 19) + ( x + 18 ) + ( x - 17 ) + ..............+ (x + 12) + ( x- 11 ) = 85$
$ x + 20 + x - 19 + x + 18 + x - 17 + .... + x+12 + x - 11 = 85$
$10 \times x + (20-19+18-17+....+12-11) = 85$
$10 \times x + (1 + 1 + .....+1) = 85$
$10 \times x + 5 = 85$
$10 \times x = 80$
$x = 8$
Vậy $x=8$
