I suppose, it is in many, if not all such textbooks. I don't know if there's a web page dedicated to this sort of thing ...
Try to work with the second part, to start with. It says the unknown sample's turbidity "is no greater than" and then asks for "the maximum" value. If it had instead said, "the unknown gives a spectrophotometer reading of exactly 0.1000, and the 10 ppm standard solution gives a spec reading of exactly 0.1000", wouldn't you know what to do?
For an example of an old post on this forum, where this procedure had to be done, repeatedly, to solve the problem, check out this old post:
http://www.chemicalforums.com/index.php?topic=25535.0These sort of questions come up often on this board, working with concentrations can stymie even experienced chemists. Try to practice the basics some more, until it becomes second nature.