Frances,
I assume you mean a cylindrical tank.
The volume V of a circular cylinder of radius r and height h is
V = r^{2} h
Your cylinder has radius 5 feet and height 15 feet so the volume is
V = 5 ^{2} 15 = 1178 cubic feet = 1178 12^{3} = 2 035 752 cubic inches.
An American gallon is 231 cubic inches so your tank holds
^{2035752}/_{231} = 8813 gallons.
There are 42 gallons of oil in a barrel so your tank holds
^{8813}/_{42} = 210 barrels.
Penny
