The volume of the cylinder is

The diameter of the cylinder is 25 feet, then
The radius of it = 1/2 x diameter

Since the height is 4.5 ft
Substitute them in the rule above

Now we will change the cubic feet to gallons

Then multiply the volume by 7.48 to find the number of gallons

Now let us divide the number of gallons by 3000 to find how many gallons of liquid chlorin he needs to buy

Then he has to buy 6 full gallons