20/a<4/5
ta có 4/5=4x5/5x5=20/25
suy ra số tự nhiên a là 25
Vì \(\frac{4}{5}=\frac{20}{25}\)nên \(\frac{20}{a}\le\frac{20}{26}\)
\(\Rightarrow a\in\left\{26;27;28;29;30;...\right\}\left(a\in N\right)\)
\(\frac{4}{5}=\frac{20}{25}\)
\(\frac{20}{a}>\frac{20}{25}\)
\(\Rightarrow a>25\)