Welcome to the enquiries forum. this is the place to ask questions relating to our books, our courses or the school. If you have a more specific problem relating to aviation theory, check out the Question and Answer forums. That's the best place to post your technical questions.
If the answer on page 134 is not sufficient, I suggest to calculate 3 scenarios:eg. dist 100nm, ias 100kt, then: 1. no wind, 2. tailwind 30kt, 3. tailwind 10kt, then compare what happens to ETP and PNR for each scenario.