



 
Jeff, The volume of a cylinder is
where r is the radius, h is the height, or length it its lying down, and π is approximately 3.1416. The length and radius need to be in the same units and I am going to use inches since I know that 1 US gallon is 231 cubic inches. There are 12 inches in a foot so the length of the tank is 120 inches. Is your tank really 45 inches around or is it 45 inches across? The circumference of a circle (the distance around) is given by 2 π r so if your tank is 45 inches around then
and hence
Thus the volume is
If the tank is 45 inches across then r = 45/2 = 22.5 inches and the volume is
Once you have the volume in cubic inches divide by 231 to obtain the volume in gallons.
I hope this helps,  


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