$a)$

$0,\left(4\right)=0,44444\dots$

$x=0,4444\dots$

$10x=4,4444\dots$

$10x-x=4,4444\dots -0,4444\dots$

$9x=4\mid :9$

$x=\frac{4}{9}$

$b)$

$0,8\left(3\right)=0,833333\dots$

$x=0,83333\dots$