Hi, In chemistry, how do you determine net volume?

Hi,

In chemistry, how do you determine net volume? I thought it was volume after adding more of a substance minus initial volume, but the answers given in this lab aren't doing that. It looks to me like the person was calculating average and not net.

Example: the lab started with 30.00mL of water. We then added 5.000g of iron to get 30.64mL. To get the net, wouldn't you do 30.64-30.00?