The back-ground reading is the reading caused by cosmic rays and this not the object.
Therefore, the back-ground reading has to be 16 as the total sum of all the detector readings always ends up as 16. You need to remember here, that the object is not radiating anything. So, it is simply asking you about its absorption.
The second question is trying to ask you about the absorption ratio. In order to find that, you need to take away the cosmic radiation, and then find the ratio of the voxels.
So take away 16, and then do a ratio of all the present values.
