Answer :
To solve this problem it is necessary to apply the concepts related to the geometry of a cylindrical tank and its respective definition.
The volume of a tank is given by
[tex]V = \frac{\pi d^2}{4}h[/tex]
Where
d = Diameter
h = Height
Considering that there are two stages, let's define the initial and final volume as,
[tex]V_0 = \frac{\pi d^2}{4}H[/tex]
[tex]V_f = \frac{\pi d^2}{4}h[/tex]
We know as well by definition that
[tex]1gal = 3.785*10^{-3}m^3[/tex]
Then we have for the statement that
[tex]V_f = V_0 -1gal[/tex]
[tex]V_f = V_0 - 3.785*10^{-3}[/tex]
Replacing the previous data
[tex]\frac{\pi d^2}{4}h = \frac{\pi d^2}{4}H- 3.785*10^{-3}[/tex]
[tex]\frac{\pi (3.6)^2}{4}h = \frac{\pi (3.6)^2}{4}(2)- 3.785*10^{-3}[/tex]
Solving to get h,
[tex]h = 1.99963m[/tex]
Therefore the change is
[tex]\Delta h = H-h[/tex]
[tex]\Delta h = 2- 1.99963[/tex]
[tex]\Delta h = 3.7*10^{-4}m=0.37mm[/tex]
Therefore te change in the height of the water in the tank is 0.37mm