Answer: the water level rises by 0.025ft each min
Explanation:
The volume of a rectangular fish tank is:
V = height*base
then if the hwight is 1 ft, we have that the volume is:
V = 1ft*20ft^2 = 20ft^3
Now, the amount of water that enters in the tank is r = 0.5ft^3/min
now, the change in the heigth will be
change in volume = r =change in height*base
r/base = height.
(0.5ft^3/min)/20ft^2 = change in height = 0.025ft/min
This means that the water level rises by 0.025ft each min