I'm having trouble understanding the scenario for LG#4 (written below). My main concern is that the scenario does NOT seem to suggest that each variable must be used. My interpretation of the scenario, in other words, is that there is no minimum for the number of times a variable must go, and therefore, that it is possible for some variables to be omitted from a hypothetical set-up.
Can you review the scenario and explain if my interpretation is right or wrong?
There are exactly three recycling centers in Rivertown: Center 1, Center 2, and Center 3. Exactly five kinds of material are recycled at these recycling centers: glass, newsprint, plastic, tin and wood. Each recycling center recycles at least two but no more than three of these kinds of material. The following conditions hold:
Any recycling center that recycles wood also recycles newsprint
Every kind of material that Center 2 recycles is also recycled at Center 1
Only one of the recycling centers recycles plastic, and that recycling center does not recycle glass
