Answer
$$t=5$$
Work Step by Step
We are given the following equation:
$$ s=4t+16t^2$$
Thus, plugging in $420$ for $s$, we find:
$$ 420=4t+16t^2 \\ 16t^2+4t-420=0\\ t_{1,\:2}=\frac{-4\pm \sqrt{4^2-4\cdot \:16\left(-420\right)}}{2\cdot \:16}\\ t=5$$
Note, t can only be positive.
