Class Openers High School

How I Stumbled Into Math Modeling Without Even Realizing It.

We started a unit on counting principles this week in my 9th grade honors class – permutations, combinations – eventually leading to the binomial theorem.  Since my  classes had used Desmos Activity Builder a few times and were familiar with the need to enter a 5-character code to start an activity, I planned to ask the following question as a class opener:

How many different 5-character DesmosActivity Builder codes exist?


This problem would have likely met my intended goal of having kids think about the fundamental counting principle in a real-world context.  It also would have taken about 10 minutes of class time, and have been forgotten about by the next day.  It felt like I was missing an opportunity to develop a deeper discussion.  A slight tweak to the question added just the right layer:

Activity codes for Desmos Activity Builder currently have 5 characters, as shown here.  When will Activity Codes need to expand to 6 characters?

And now we have a problem which requires a bit more than a quick calculation.  To start, I asked students to work in their teams to make a list of information they would need to help solve this problem.  This was not easy or comfortable for them – but a preliminary list of questions emerged from group discussions:

  • How many 5-character codes are there?
  • Are codes used less on weeekends and summers?
  • Can letters repeat in codes?
  • How many codes a day are used?

This was a good start to set kids in motion to think about how to solve the problem.  I’m hoping they will think about new questions or revise their questions as we go along…the class did not disappoint!


As kids worked, clarifying questions came up – some of which I just didn’t know the answer to, and hadn’t really thought about:

Mr. L, are there any zeroes in codes? Kids might confuse them with the letter O.

Mr. L, I don’t see any L’s in the codes?

Excellent observations, and restrictions we need to think about in our calculation. A tweet to the Desmos crew lent some clarity, and added more restrictions!

Thank for the intel, Eli!


This was tricky for my class. To help, I reminded students that when we started the semester, codes were 4 characters.  When did the Desmos 5-character era begin?  A quick scroll through my history (shown here) provides some info. After further interrogation from my class, I shared that Activity Builder started around July of last year with 4-character codes.  Add this to our bucket of helpful info.



Writing a draft solution was the next task for students.  But instead of turning it in to me immediately, I formed class teams of 3 where students shared their drafts and ideas.  I used this opportunity to build teams of students who I observe don’t often interact or chat.  From here, I gave students another day to think about their explanation – keeping in mind that there are no right answers to this question, only answers we can defend. But it still feels like we are missing a key piece in this problem……


The next morning as students were mingling before the bell, I looked across the room at the laptop of Jacob – one of my more insightful, but also introverted, students:


It’s the mother lode!

The google trends graph for student.desmos.  Yes! Yes! Yes!  Stop everything kids, we need to talk!  Jacob – tell us all about this graph. How does this new info factor into our estimates?  What should we do with it?  Is this going to continue?  And with this, I gave the class an extra day to think about their responses, share, and dig deeper.  And while many students simply estimated a growth rate by doubling or tripling their computed rate (this is fine with me), I am getting some responses which far exceed my expectations – like Jacob, who developed a growth function and evaluated integrals (did I mention this is a 9th grade class????)


Yep, this was definitely better than my originally intended problem!



High School Statistics

Compute Expected Value, Pass GO, Collect $200

Photo Oct 11, 7 08 49 AM.jpgExpected Value – such a great time to talk about games, probability, and decision making!  Today’s lesson started with a Monopoly board in the center of the room. I had populated the “high end” and brown properties with houses and hotels.  Here’s the challenge:

When I play Monopoly, my strategy is often to buy and build on the cheaper properties.  This leaves me somewhat scared when I head towards the “high rent” area if my opponents built there.  It is now my turn to roll the dice.  Taking a look at the board, and assuming that my opponents own all of the houses and hotels you see, what would be the WORST square for me to be on right now?  What would be the BEST square?

For this question, we assumed that my current location is between the B&O and the Short Line Railroads.  The conversation quickly went into overdrive – students debating their ideas, talking about strategy, and also helping explain the scenario to students not as familiar with the game (thankfully, it seems our tech-savvy kids still play Monopoly!).  Many students noted not only the awfulness of landing on Park Place or Boardwalk, but also how some common sums with two dice would make landing on undesirable squares more likely.


After our initial debates, I led students through an analysis, which eventually led to the introduction of Expected Value as a useful statistic to summarize the game.  Students could start on any square they wanted, and I challenged groups to each select a different square to analyze.  Here are the steps we followed.


First, we listed all the possible sums with 2 dice, from 2 to 12.

Next, we listed the Monopoly Board space each die roll would causes us to land on (abbreviated to make it easier).

Next, we looked at the dollar “value” of each space.  For example, landing on Boardwalk with a hotel has a value of -$2,000.  For convenience, we made squares like Chance worth $0.  Luxury Tax is worth -$100.  We agreed to make Railroads worth -$100 as an average.  Landing on Go was our only profitable outcome, worth +$200. Finally, “Go to Jail” was deemed worth $0, mostly out of convenience.

Finally, we listed the probability of each roll from 2 to 12.

Now for the tricky computations.  I moved away from Monopoly for a moment to introduce a basic example to support the computation of expected value.

I roll a die – if it comes out “6” you get 10 Jolly Ranchers, otherwise, you get 1.  What’s the average number of candies I give out each roll?

This was sufficient to develop need for multiplying in our Monopoly table – multiply each value by its probability, find the sum of these and we’ll have something called Expected Value.  For each initial square, students verified their solutions and we shared them on a class Monopoly board.


The meaning of these numbers then held importance in the context of the problem – “I may land on Park Place, I may roll and hit nothing, but on average I will lose $588 from this position”.

HOMEWORK CHALLENGE: since this went so well as a lesson today, I held to the theme in providing an additional assignment:

Imagine my opponent starts on Free Parking.  I own all 3 yellow properties, but can only afford to purchase 8 houses total.  How should I arrange the houses in order to inflict the highest potential damage to my opponent?


I’m looking forward to interesting work when we get back to school!

Note: I discussed my ideas about this topic in a previous post.  Enjoy!

High School Technology

Pulling In To the Station

My school isn’t 1-1 with technology yet, though there are rumblings we will get there next year….or the year after….or 2031…anyway, it’s time to get techy!  My new classroom features 4 computer stations in the back – nice to have, but not super-helpful with classes of about 24 each. Station-model classroom structure has been super-helpful in my pre-calculus class in the first month. Besides the chance for all students to participate in rich technology-based activities, I’ve had the opportunity to carve out valuable small-group time with students.  Here’s an example:

In our first pre-calc unit, we review functions and their shirts, folding in new ideas like the step function, piecewise and even/odd functions.  My objective for the class was for students to consider functions in varied forms.  As students entered class, playing cards were drawn to establish their groupings, so there were 3 groups of 7 or 8.  With 15 minutes on the classroom clock, students started on their first station:

  1. Group 1 gathered in a small group with me in a circle of desks, where we worked through proving functions even or odd, and sketching their graphs.
  2. marbleslideGroup 2 worked at the computer stations on a Desmos Marbleslides featuring quadratic functions, with many students pairing up to work together. If you have never tried a Marbleslides, run and play now – we’ll wait for you to come back…
  3. Group 3 worked out in the courtyard (hey, my new classroom leads outside – which is nice) on a group task involving a piecewise function.

After groups had rotated through all 3 activities, we had time to recap / share and assess our learning over the hour.  Here’s why I need to do this more:

  • The small group station let me touch base with every student, assess strengths, find out what we need to work on, and provide feedback to everyone.
  • Marbleslides is sneaky awesome! When students begin to obsess over function shifts and how to restrict domains and don’t want to peel away from their computer, you know something is going right.
  • Class went fast! It felt like the mixed practice from Let It Stick was now becoming part of my classroom culture.
  • My pre-calc is mostly 11th and 12th graders, who have had a pretty traditional classroom experience in their math lives.  I can sense they appreciate that something difference is happening.
  • All students are responsible for their learning.  Even the least-active task, the piecewise function, was used the next class for sharing out and a jumping-off point.