It takes 8 hours to drive to Joliet, and that's 6 hours more than half the time it takes to drive to Davenport. So, I did 8-6 which equals 2. So 2 hours is half the time it takes to get to Davenport, which means it takes 4 hours to get to Davenport.
If you plug in the radius (0.5 because the radius is half of the diameter) and the height to the formula for volume of a cylinder you will get pi•0.5 squared•15 which when solved is 11.775.