No More Double-Nots

As I wrote recently, I have been trying to find a better way to explain the important features of a “/A->B” rule in a grouping game. The “/A->B” and “A->/B” rules can be literal “game changers” in any in/out, sorting, or matching game, but they are extraordinarily hard to explain. A student has to understand conditional reasoning thoroughly and have an almost intuitive grasp of how the necessary and sufficient terms work to be able to read either of those rules and see the implications in a grouping game. Most people who are that good at conditional logic don’t need me to tutor them.

PowerScore uses a special “double not arrow” (“<-|->”) to represent one of these rules, but I find this arrow hard to use and impossible to teach.  I have searched and searched for a better way to teach this concept, without any success, so finally decided I would make up new symbols for the “/A->B” and “A->/B” rules. Within a few hours of posting my idea, somebody suggested something even better (which proves that you can’t find everything you need to know by searching for it).

This helpful commenter reminded me that any conditional statement (“A->B”) can be written as an “or” statement (“/A or B”). This is something I teach all my students to help them understand why “unless,” “except,” “without,” and “until” all have a “not” in them. (“You won’t pass unless you understand logic can be written either as “/P or L” or as “P->L.”) That means that “/A->B” can be rewritten as “//A->B,” or, if you take out the double negative, “A or B.”

“A or B” is really easy to work with. In a logic game, I just write down A/B and I know exactly what to do with it. Each slot must have an A, or a B, or both. (You must always remember, of course, that “A or B” means “A or B or both.”)

What about the other rule, “A->/B”? The same rewriting approach gives us “/A or /B.” That works out to be the same as “NOT (A and B).” On a logic game, I write that as an [AB] block with a slash through it. I can’t have A and B together. If I’m working on an in/out game, that means at least one of those has to be “out,” so I can write “A/B” over in my “out” column.

If you can remember that these two rules produce “A/B” or “/[AB],” you’re already way ahead of most other test-takers. If you can remember which is which, you have a significant strategic advantage. Most grouping games with one of these rules in them have one or more questions that directly depend on them. I have seen grouping games with three questions that can be answered in 15 seconds or less if you start with “A/B” in the right slot.

So–how do you remember which is which? “/A->B” means that if you don’t have A, you must have B. That’s exactly what an “or” statement says. If you can remember that much, you can instantly rewrite “/A->B” as “A/B” every time. If you can’t you have to ask, “So what happens if A is ‘in’? I guess B can do whatever it wants. So  A and B can both be ‘in,’ so that means ‘A/B.'”

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