a) $x - 105 : 21 = 15$
$x - 5 = 15$
$x = 15 + 5$
$x = 20$
b) $\text{ (2x - 1) . $5^{4}$ = 3 . $5^{5}$}$
$\text{(2x - 1) . 625 = 3 . 3125}$
$\text{ (2x - 1) . 625 = 9375}$
$\text{2x - 1 = 9375 : 625}$
$\text{2x - 1 = 15}$
$\text{2x = 15 + 1}$
$\text{2x = 16}$
$\text{x = 16 : 2}$
$\text{x = 8}$
c) $\text{26 + 5.$2^{x + 2}$ = 106}$
$\text{5.$2^{x + 2}$ = 106 - 26}$
$\text{5.$2^{x + 2}$ = 80}$
$\text{$2^{x + 2}$ = 80 : 5}$
$\text{$2^{x + 2}$ = 16}$
$\text{$2^{x + 2}$ = $2^{4}$}$
⇒ $\text{x + 2 = 4}$
$\text{x = 4 - 2}$
$\text{x = 2}$
Vậy $x = 2$
