Answer
$$t\approx0.21$$
Work Step by Step
$$V=\cos2\pi t$$
where $t\in[0,\frac{1}{2}]$
As $V=0.25$, we would have the equation: $$\cos2\pi t=0.25=\frac{1}{4}$$
So, $$2\pi t=\cos^{-1}\frac{1}{4}\approx1.318 (rad)$$ (be careful about radians and degrees)
$$t=\frac{1.318}{2\pi}$$
$$t\approx0.21$$
$t\approx0.21$ is the result of this question.
