Question

How do you write 5.8 as a mixed number

Answer 1

5 8/10 or it would be 5 4/5 if you want it simplified.

Answer 2

We know that
the number $5.8$
is equal to 
$5+0.80$
and
$0.80$  is equal to
$8/10$
divide by 2 both members
$\frac{8}{10} = \frac{4}{5}$

therefore

the number $5.8$  is equal to  $5 \frac{4}{5}$