Answer
The least possible value of $t$ such that $V=0$ is $0$ seconds.
Work Step by Step
$$V=\cos2\pi t$$ with $0\le t\le\frac{1}{2}$
For $V=1$, we have
$$\cos2\pi t=1$$
Here the condition states that $0\le t\le\frac{1}{2}$. In other words, $t\in[0,\frac{1}{2}]$
Also $t$ is minimum when $2\pi t$ is minimum, as $2\pi$ is a constant.
Therefore, we would be able to find the least value of $t$ as we find the first value of $2\pi t$ that would have $\cos(2\pi t)=1$ over the interval $[0,2\pi)$.
Such a value of $2\pi t$ can be found using the inverse function for cosine:
$$2\pi t=\cos^{-1}1$$
$$2\pi t=0$$
$$t=0(seconds)$$
$0\in[0,\frac{1}{2}]$ so this solution is accepted.
In conclusion, the least possible value of $t$ such that $V=0$ is $0$ seconds.
