`\text {Mon}`
$\dfrac{1}{3}$-$\dfrac{2}{...}$ `xx` $\dfrac{3}{8}$=$\dfrac{1}{4}$
$\dfrac{2}{...}$ `xx` $\dfrac{3}{8}$=$\dfrac{1}{3}$-$\dfrac{1}{4}$
$\dfrac{2}{...}$ `xx` $\dfrac{3}{8}$=$\dfrac{1}{12}$
$\dfrac{2}{...}$=$\dfrac{1}{12}$:$\dfrac{3}{8}$
$\dfrac{2}{...}$=$\dfrac{2}{9}$
⇒ Số thích hợp vào chỗ chấm là số `9`