This is short math activity that is well placed in a discussion of the four basic operations. You could use it as a mini-lesson and have your students try to come up with rules of their own. You could consider this as an activity for a substitute because it is a stand-alone activity. It does help to have some math fluency and enthusiasm when puzzling through these and working with the students.
The activity is self-explanatory, and the answer key should resolve any questions. I have included both Word and PDF files.