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$a.$ $x\times \:4,8=60$

$⇒x\times \:4,8\times \:10=60\times \:10$

$⇒48x=600$

$⇒\dfrac{48x}{48}=\dfrac{600}{48}$

$⇒x=\dfrac{25}{2}$

`Vậy` $x=\dfrac{25}{2}$

$b.$ $100\:-\:x\::\:6\:=\:77,\:8$

$⇒\dfrac{6\left(100-x\right)}{6}=77,8×6$

$⇒100-x=466,8$

$⇒100-x-100=466,8-100$

$⇒-x=366,8$

$⇒\dfrac{-x}{-1}=\dfrac{366,8}{-1}$

$⇒x=-366,8$

`Vậy` $x=-366,8$