Nobody seems to have discussed this question yet:
"...One way to tell whether or not the DNA has been completely digested is to count up the total length of the DNA fragments. Explain how scientists could use this data to conclude that the DNA has not been totally digested by the enzymes."
This completely threw me.
As I see it, either the total fragment length is equal to the length of the original DNA or it isn't.
If it is, this does not prove anything. No matter how many times you cut a piece of string, the total length of the fragments will stay the same. The total fragment length gives no indication whatsoever of whether or not there are restriction sites yet to be cut.
If it isn't, then bases have apparently vanished into thin air. Which cannot happen.
I have checked the textbook and the revision guide, and there isn't a single hint of the answer to this question. Possibly I just read it wrong.
Edit: The only way total fragment length would be of any use is if they knew how many fragments they would get to start with, in which case they don't even need to calculate total length, they would just need to count the fragments.