($\frac{3}{10}$ + x) . $\frac{1}{3}$ = $\frac{2}{5}$
$\frac{3}{10}$ + x = $\frac{2}{5}$ : $\frac{1}{3}$
$\frac{3}{10}$ + x = $\frac{2}{5}$ . 3
$\frac{3}{10}$ + x = $\frac{6}{5}$
x = $\frac{6}{5}$ - $\frac{3}{10}$
x = $\frac{12-3}{10}$
x = $\frac{9}{10}$
Chúc bạn học tốt ^^