$\text{a)}$ $\dfrac{3}{10}$ $\text{+}$ $\dfrac{2}{5}$

$\text{=}$ $\dfrac{3}{10}$  $\text{+}$ $\dfrac{4}{10}$ 

$\text{=}$ $\dfrac{7}{10}$ 

$\text{b)}$ $\dfrac{4}{15}$ $\text{+}$ $\dfrac{6}{45}$

Ta có: $\dfrac{6}{45}$ $\text{=}$ $\dfrac{6 : 3}{45 : 3}$  $\dfrac{2}{15}$

 $\dfrac{4}{15}$ $\text{+}$ $\dfrac{2}{15}$

 $\text{=}$ $\dfrac{6}{15}$   $\text{=}$ $\dfrac{2}{5}$  

$\text{c)}$  $\dfrac{48}{28}$ $\text{-}$ $\dfrac{9}{7}$ 

Ta có: $\dfrac{48}{28}$ $\text{=}$ $\dfrac{48 : 4}{28 : 4}$ $\text{=}$ $\dfrac{12}{7}$ 

 $\dfrac{12}{7}$ $\text{-}$ $\dfrac{9}{7}$ 

 $\text{=}$ $\dfrac{3}{7}$ 

`\color{DarkTurquoise}{\text{ ∘ nhminnh}`