Reservoir capacity
$V = \frac{1}{3}(A_1 + A_2 + \sqrt{A_1A_2})h$
$V = \frac{1}{3}[ \, 100^2 + 90^2 + \sqrt{100^2(90^2)} \, ](10)$
$V = 90,333.33 ~ \text{ft.}^3$
$V = 90,333.33 ~ \text{ft.}^3 \times \left( \dfrac{12 ~ \text{in.}}{1 ~ \text{ft.}} \right)^3$
$V = 156,096,000 ~ \text{in.}^3$
$V = 156,096,000 ~ \text{in.}^3 \times \dfrac{1 ~ \text{gal.}}{231 \text{in.}^3}$
$V = 675,740.2597 ~ \text{gal.}$
Volume = Discharge × time
$V = Qt$
$675,740.2597 = 200t$
$t = 3,378.7 ~ \text{min.}$
$t = 3,378.7 ~ \text{min.} \times \dfrac{1 ~ \text{hr.}}{60 ~ \text{min.}}$
$t = 56.312 ~ \text{hrs.}$ answer