## Elementary and Intermediate Algebra: Concepts & Applications (6th Edition)

$$t=5$$
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.