Answer
$80\ \ \mathrm{f}\mathrm{t}/\mathrm{s}$
Work Step by Step
By Exercise $70$, the highest point is $\displaystyle \frac{v_{0}^{2}}{64}$ feet above the starting point.
$v_{0}^{2}/64=100$
$v_{0}^{2}=64\cdot 100$
$v_{0}=8\cdot 10=80\ \ \mathrm{f}\mathrm{t}/\mathrm{s}$
