Question

How do u decompose 4/5

Answer 1

You can not decompose 4/5 because it is not possible. If you try to decompose 4/5 then you would get a negative answer or an error.
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Answer 2

You mean simplify? 

You can't simplify it anymore because the numerator is an even number and the denominator odd..

But,

$\frac{4}{5}$ = 0.8

And equivalent fraction:

Multiply 2 from the numerator and denominator

$\frac{4~multiply~by~2}{5~multiply~by~2} = \frac{8}{10}$