Ok
Data
h = 12 ft
r = 3ft
water = 1 foot deep
pi = 3.14
Speed = 20 ft^3/min
Formula
Volume of a cylinder = pi*h*r^2
Substitution
Volume of a cylinder = 3.14*11*3^2
Simplification
Volume of the cylinder = 310.86 ft^3
-Calculating the time
310.86 ft^3 ---------------------------- x
20ft^3 ---------------------------- 1min
x = (310.86 x 1) / 20
x = 15.54 min
It will take 15.54 min to fill in the tank