For 1/15, the clearer question might be:

In the Community, you get two pets. There are six choices of pets to have: dog, cat, fish, snake, bird, and hamster. The Elders pick the pets for each family without any duplication (e.g. no cat-cat pairs). What is the probability of getting a dog and a cat if the order doesn't matter (i.e. cat-dog is the same as dog-cat)?Typically, this is how the preservice teachers read the original problem even though the original lacks many of the specifics. They see it as a combination problem and so add the necessary conditions in their head. It helps these future teachers to be aware of the implicit conditions hiding in many problems.

In the Community, you get two pets. There are six choices of pets to have: dog, cat, fish, snake, bird, and hamster. The Elders pick the pets for each family without any duplication (e.g. no cat-cat pairs). What is the probability of getting a dog first and then a cat?Because the preservice teachers are familiar with permutations this revision is fairly simple for them.

In the Community, you get two pets. There are six choices of pets to have: dog, cat, fish, snake, bird, and hamster. The Elders pick the pets for each family. Duplications, cat-cat pairs, are possible. What is the probability of getting a dog first and then a cat?

This, too, comes quickly since they are comfortable with the multiplication principle.

In the Community, you get two pets. There are six choices of pets to have: dog, cat, fish, snake, bird, and hamster. The Elders pick the pets for each family. Duplication, cat-cat pairs, are possible. What is the probability of getting a dog and a cat if the order doesn't matter (i.e. cat-dog is the same as dog-cat)?

While they come to this version of the problem easily based on the prior revisions, this is a new context for them and they are eager to explore it further.

Before we look at the math, I engage the preservice teachers in a discussion of the pedagogical worth of having "messy" problems with many possible interpretations. They are inclined to want to clean up problems before sharing them with students but they recognize this is often based on their own experiences in math class. Fortunately, there are usually some voices that identify how considering different points of view made the problem much richer. This ability to identify underlying conditions and considering the alternative problems supports the "Now what?" stance I hope to foster.

We explore the "Now what?" question regarding how to generalize the "1/21 case" in the next post.

Interesting. I admit the first lesson I drew out of it was "see, be careful how you phrase problems lest they be misinterpreted," but I like your conclusion much better. I could imagine some great lessons with intentionally messy problems and having students work on and interpret them in groups.

ReplyDeleteThanks, Grace. To be honest, I am also drawn to developing the problem that cannot be misinterpreted. It's how I was taught - one correct answer. Fortunately, I have begun to trust my learners more and more. With their help I have found there is more interesting territory to explore than the single path afforded by the perfectly written problem.

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