I will discuss what placeholders are, and how Mystery Master uses them to solve particularly hard logic puzzles. A placeholder is a noun where the name is not given. If placeholders are required, they are usually for one noun type where the values for one or more of the nouns are unknown. You must have rules that can calculate the values of the placeholders.
Puzzles that have placeholders are given in the following sections.
For this puzzle, placeholders are required for the nouns of the "Attendance" noun type. The initial names for the attendances are: A1, A2, A3, A4, and A5. Each attendance is assigned to a talk, where the first talk has attendance A1, the second talk has attendance A2, and so on. From clue 2, the only known attendance is 24, which is the largest number. And because of clue 6, this value must be assigned to either A4 or A5. Clue 10 also tells us that the attendances are all different.
Please apply common sense as to what values are appropriate for attendances. You can't have a negative number of people in attendance; you must have whole numbers. But here's a question for you: "Can you have zero attendance?"
If I was snarky I would ask "How should we count half-wits?" Fortunately, I am not. Snarky, that is. The jury is still out if I'm half-witted.
For this puzzle, placeholders are required for the nouns of the "Age" noun type. The initial names for the ages are: A01, A02, all the way to A12. Each age is assigned to a month, where the age of the January winner is A01, the February winner is A02, ..., and the December winner is A12. The first clue tells us all of the ages are different. The ages that are given in the puzzle are: 20 (clue 6), 50 (clue 8), 46 (clue 17), and 26 (clue 18). The rules need to include these numbers when calculating the ages.
You can assume that an age cannot be negative, or even zero. But should you make any other assumptions? Should child labor laws apply to a logic puzzle? Would Methuselah work in a used car lot? You tell me.
In both puzzles, each placeholder is assigned to a noun that is already sorted within its noun type. For example, the talks are sorted by who talked first, second, and so on. And the months are in calendar order. After each mark is entered, the rules need to be invoked to update the placeholders.
I have to admit that managing placeholders can be quite the challenge.