Answer
$\frac{16}{100}$
Work Step by Step
$x^2 - \frac{4}{5}x$
The coefficient of the x-term in $x^2 - \frac{4}{5}x$ is $-\frac{4}{5}$.
Half of $-\frac{4}{5}$ is $-\frac{4}{10}$, and $(-\frac{4}{10})^2 = \frac{16}{100}$
Add $\frac{16}{100}$.
The result is a perfect square trinomial.
$x^2 - \frac{4}{5}x + \frac{16}{100} = (x -\frac{4}{10})^2$
