Dear Kerry: Rule Substitution Questions

Hello Kerry,

I am getting to a point where I can answer everything but the “substitution” question in logic games. Could you tell me the best method to tackle these types of questions?

Regards,

Pooja

Hi Pooja,

A lot of people (maybe even most!) have trouble with rule substitution questions in Logic Games. I think these questions have gotten a bad rap, though. Sometimes they can be very hard, but often they can be pretty easy. So I think it is worth at least spending a short time considering them, rather than skipping them, as some people do.

The first thing I think about with one of these questions is whether the rule you are getting rid of has a variable in it that is in a tight relationship with another variable, such that I can imagine just substituting that other variable in. For example, you have a game with these two rules:

• A is before B.

  1. B is immediately before C.

If the rule that the substitution question wants to get rid of is “A is before B,” I would think: Well, B is in a block with C. So if I said, “A is before C,” or “C is after A,” it would have exactly the same effect.

It would be the same approach if I had a grouping game with rules like this:

• A and B are on the same team.

  1. A and C are on different teams.

If I need to get rid of the rule, “A and C are on different teams,” I could easily imagine a substitute rule saying, “B and C are on different teams.”

This approach works surprisingly frequently, but it certainly doesn’t work all of the time. Other things to think about in trying to predict an answer to a rule substitution question are:

• If the rule you are getting rid of is a conditional rule, consider whether one of the answer choices is a contrapositive of that rule.

  1. In an ordering game, if the rule you are getting rid of orients a variable toward one end of the diagram (e.g., A must be one of the first three packages delivered), consider whether one of the answer choices orients that letter toward the other end of the diagram (e.g., A may not be one of the final three packages delivered).

  2. Similarly, if the rule you are getting rid of eliminates a lot of possibilities (e.g., A may not be paired with B, C, or D), consider whether one of the answer choices instead tells you what is possible (e.g., A may only be paired with E or

     F).

The bottom line is, the correct answer to a rule substitution question has to be a perfect substitute– with the “new” rule in place, every possible solution that worked under the old rule has to continue to work under the new rule, and every possible solution that did not work under the old rule cannot work under the new rule, either. The new rule needs to be exactly as restrictive as the old one–no more, no less.

Thanks for the question,

Kerry