One of the many tasks I perform for my school district is serving as math club sponsor. While I often attempt to find interesting activities and experiences for the club, many students join math club to participate in the contests we tackle each year, and this month tends to be a particularly busy period for contests. In-house contests include the Pennsylvania Math League and the American Mathematics Competition exam series. Today, I am writing my blog post from a lecture hall at Lehigh University, while two floors below hundreds of students are participating in an annual contest organized by Dr. Don Davis, who uses the event to recruit students for his American Regions Math League teams. Tomorrow, two teams of students I work with will attempt the Moody’s Math Challenge, where students are given 14 hours to complete and open-ended question and submit a solution.
I recall a conversation I had with a math department head I worked with when I first became a teacher, and the conversation centered around why our school didn’t have a math club. The veteran teacher responded that he didn’t believe in doing math problems as competition, and I suppose that I agree with the essence of his argument: that sitting alone, isolated, doing a series of problems may not be the most enriching of pursuits. But the conversations that take place surrounding challenging problems can lead in interesting directions which often encourage collaborative thinking and build confidence in approaching “scary-looking” problems.
The problems from the AMC 10 and 12 exams, in particular, lend themselves to discussions of problem-solving approaches. Each exam is set up with 25 multiple-choice questions which loosely go in sequence from least to most challenging. Many students I coach can handle the first 10 to 12 questions, and may venture as high as question 20, before sensing that the questions have taken a turn towards the evil…questions with wording and symbols beyond their experiences.
We can use contest questions to encourage not only higher-level thinking from all of our students, but also develop persistence in problem solving. Consider the following problem, which was #15 on this years AMC-12:
A 3×3 square is partitioned into 9 unit squares. Each unit square is painted either white or black with each color being equally likely, chosen independently and at random. The square is then rotated 90 degrees clockwise about its center, and every white square in a position formerly occupied by a black square is painted black. The colors of all other squares are left unchanged. What is the probability that the grid is now entirely black?
This is a problem 15 from the contest, which implies that it is bordering on the medium to hard-type of problem. I like this question for two reasons:
- It’s a probability question. I know that these types of problems often appeal to me, as opposed to geometry questions, which often interest me less.
- It has an accessible premise. While we may have trouble down the road computing the probability, this problem can be easily de-constructed, simulated and discussed, even by middle-school students.
We can walk through this problem by giving students some 3×3 grids and a black marker. According to the problem, each square is painted white or black at random. Have students make their own grids, then make a copy of it, which will then be rotated 90 degrees:
Then, follow the directions to make an altered grid: every white square in a position formerly occupied by a black square becomes black. All other squares are left unchanged. In the example below, our grid fails, since the results is not all black.
Where can we head from here? Depending on the maturity and sophistication of the students, there are a few paths to consider:
- With younger students, hand out some pre-made grids, where some will become all black after the transformation. Can students categorize those which become all black?
- For more sophisticated students, hand out more 3×3 grids and experiment to see if they can develop one or more grids which satisfy the problem.
For some students, the problem may stop here, which is fine. The experience of having tackled part of a complex problem is a success unto itself. You can even let the problem stew with students for a few days to discuss with friends and parents before reaching some conclusions. In this problem, there are 3 dependencies, 3 different aspects of the grid to consider:
- The center square must be black
- The corner squares (A,B,C,D, below) must meet certain arrangements
- The non-corner squares (e,f,g,h, below) must also meet certain arrangements
Can we list arrangements for ABCD which will result in all black? Certainly black-black-black-black works, but so does white-black-white-black. Are there others? This then shifts the nature of the problem from a scary-looking probability question to a more tame (but still semi-scary) counting problem. I’ll leave the counting to you and your students.
In my next post, we’ll look at a geometry example from this year’s AMC-12.
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