wallsare 3 inches thick
diameter is 20 feet, find real diametter
legnth is 50 feet, find real legnth
minus 6 inches from each measurement since that is not included
20ft-6in=19.5ft
50ft-6n=49.5ft
we have a cylinder and a 2 hemispheres
2 hemispheres=1 sphere
Vcylinder=hpir^2
Vsphere=(4/3)pir^3
r=d/2
r=19.5/2=9.75
h=49.5
Vcylinder=49.5*pi*9.75^2=14783.058738886 ft^3
Vspere=(4/3)pi9.75^3=388419466778 ft^3
add them
Vcylinder+Vspher=14783.058738886+388419466778 ft^3=18665,478205664ft^3
V=18665,478205664 ft^3
times 7.48 to find gallons
Vingallons=139617.77697837 gallons