`4,` 

`a)`

$\dfrac{16 × 48 + 16 × 24 + 16 × 28}{325 - 317 + 426 - 418}$

`=` $\dfrac{16 × ( 48 + 24 + 28 )}{( 325 - 317 ) + ( 426 - 418 )}$

`=` $\dfrac{16 × 100}{8 + 8}$

`=` `1600/16`

`=` `100`

`b)`

`9/12` `=` `1/4` `+` `1/3` `+` `1/6`

`=>` Vì`:` `1/4` `=` $\dfrac{1 × 3}{4 × 3}$ `=` `3/12`

               `=>` `1/4` `=` `3/12`

               `1/3` `=` $\dfrac{1 × 4}{3 × 4}$ `=` `4/12`

               `=>` `1/3` `=` `4/12`

               `1/6` `=` $\dfrac{1 × 2}{6 × 2}$ `=` `2/12` 

               `=>` `1/6` `=` `2/12`

`=>` Mà`:` `4/12` `+` `3/12` `+` `2/12` `=` `9/12`

Nên`:` `9/12` `=` `1/4` `+` `1/3` `+` `1/6`