The class of problem they use involves ambiguous or deduplication-style constraints - how do you track behaviour acrossva system when you can't be certain which identifier belongs to which entity? How do you count or aggregate events when the key you've been given might mean one of several things?
You have to track user page visits, but there are two unique identifiers that can be used - browser ID or IP address.The ID you get given, you don't know which of the two it corresponds to. You have to count the amount of visits to each page for the user using this ambiguous ID.