The formula for solving the volume of a tank is V=pi*r²*h where "r" represent the radius and "h" for the height.
In the given problem we need to solve for the volumes,
V=3.1416*r²*h
r=diameter/2
r=2.5/2
r=1.25
h=3.5-1.5 since 1.5 is already filled with water
h=2
V=3.1416*1.25²*2
V=9.8175 ft³
Solving for how many more gallons (1 cubic feet is 7.5 gallons)
# of gallons=9.8175 *7.5/1
# of gallons=73.63 gallons