Paul,
I assume that the tubes are 12 in in diameter and 48" long.
The volume of a cylinder is
r^{2} h
where r is the radius and h is the height. You have the dimensions in inches so first you need to convert them to yards. There are 36 inches in a yard so
r = ^{12}/_{2} = 6 inches = ^{6}/_{36} yards and h = ^{48}/_{36} yards
Thus the volume in cubic yards is
( ^{6}/_{36} )^{2} ( ^{48}/_{36} ) = 0.116
But you have 15 such cylinders so you need
15 0.116 = 1.75 cubic yards of concrete.
Penny
