Currently, that is the only question under this tag, but these questions would also fit under what I think the intention of the tag is:
- Ten numbers on a blackboard
- Numbers on a Blackboard
- The last number on the blackboard
- Triangle of numbers on a blackboard
This type of problem is characterized by a set of numbers being written on a blackboard, and operations being used to change the numbers. The solution usually involves an invariant or monovariant of the system. As xnor commented:
I want to point out the general idea here that also solves this problem: apply a transformation to the numbers so that the combining operation on the transformed numbers is commutative and associative.
Two of the questions are also tagged reachability: "A puzzle on a discrete system where one has to decide whether a certain system state can be reached through a finite number of steps." This seems like the closest tag we have currently, but it is broader than the category of "blackboard questions".