Better Than a Double-Not

I teach my students that as soon as they realize a logic game is a grouping game they should start tapping one foot and saying “count… count… count” in the back of their minds. Grouping games are all about counting. The instant you can fill up any group, you have usually solved the problem.

This priority on counting makes information about slots that have to be full (or empty) especially valuable. Two common conditional rules provide just that kind of information. They are:

• A->/B
• /A->B

The “/A->B” rule is so important for grouping games that PowerScore uses a special symbol (“A<-|->B”, or “the double-not arrow”) to note it. As a person who understands how important this is for grouping games, I think the “double-not arrow” is brilliant. As a tutor who wants to explain it to my students, I think it is both frustrating and confusing. That’s why I have come up with two arrows of my own.

Before I unveil my new arrows, let’s see why it is so important to spot the “double-not arrow” situation and so confusing to use it. Let’s walk through what “/A->B” really means. In a grouping game, we tend to think of items as “in” or “out” rather than “true” or “false,” so we’ll use in/out terminology for this discussion.

• If A is OUT, B is IN means:
  • Either A or B must be IN
  • A and B cannot both be OUT
  • A and B can both be IN

This means that any in/out game with a “/A->B” rule will always have either A or B “in.” By the same logic, and in/out game with the other rule (“A->/B”) will always have either A or B “out.” That is essential information! But how do you teach that to a student who is still trying to figure out the basics of conditional reasoning?

I have searched the Internet looking for clues. This is a PowerScore symbol, so I figured they must have a way to teach it. If they do, they haven’t printed it or posted it yet. Instead, they have forum discussions where they try to help people untangle themselves after they get it all confused–which is what I have been doing. Up until now.

What we need here are some simple symbols that make this easy and obvious. Fortunately, not only can we come up with such symbols, we can write them out with a keyboard. Note how the slash comes first in the “/A->B” situation, but comes second in the “A->/B” case. Let’s turn those slashes into pictures. If we put the forward slash first, we can make a “/\” picture. If we put it second, we get a “\/” picture.

• /A->B turns into A<-/\->B
• A->/B turns into A<-\/->B

Pictures are helpful if they mean something, so let’s call the “/\” picture an “erupting volcano.” The “erupting volcano arrow” means that something is erupting, so that something must be in your slot. The “\/” looks like a “leaky funnel,” which means something is leaking, which means something must be out.

If you can remember that “slash comes first” means “/\,” and “/\” means “erupting volcano,” and “erupting volcano” means something must be in, you can turn a “/A->B” rule into a full slot within seconds. And if you can remember what a “leaky funnel” does, you’ll fill an out slot just as fast.

And… if you’re tapping your foot, saying, “count… count… count” in the back of your  head, that full or empty slot just made the game much easier!