I had the same problem at first. (That's why I ended up here.
After some time I finagled it out though:
Each "exercise" in the worksheet corresponds to one line in the ALU-truth-table.
So the first example, where the solution is given, corresponds to the line where f(x,y) is supposed to yield 0.
The next where f(x,y) yields 1 ... and so on.
(And rereading the text in the alu-worksheet it says so right from the beginning
BTW: Like probably many others in the forum I'm going through the book all on my own - with no background knowledge in EE or such. I therefore absolutely depend on the forum. Without you guys I probably would have been lost already in ch. 1. So - thanks a lot for all the summaries, helpful hints, tables and other tools you come up with!!!