The best 12,000 rupiah I've spent this year

Earlier this year, while waiting for a train in Yogyakarta, this caught my eye:

A wonderful lesson plan for only 12,000 rupiah!
Click on the image for a larger view.

What really intrigued me was the small Arabic numbers above each Western number - even more interesting, the numbers didn't match:

I knew I had struck gold! 

A few weeks later in my Year 7 class, I split up the calendar pages, and put two sheets on each table group.  The class had recently done a lesson on ancient Egyptian and Roman numbers and heard that we use Indo-Arabic numbers - although at this stage they hadn't seen modern Arabic numbers. "Your mission", I told them, "is to find five interesting things about this calendar- and can you tell me where I bought it?". I let them at it, thinking this would be a ten minute activity. How wrong I was - the activity lasted fifty minutes including a whole class discussion at the end - I was surprised how involved the students were, and how much quality mathematical discussion resulted from their findings and their questions.

At first students focused on the obvious : the Arabic numbering system. Most noticed there was something 'wrong' with the numbering - they didn't match.  I asked a few more questions to groups who hadn't seen the disconnect between the two sets of numbers. One student could read the Arabic text and recognised the names of the Muslim calendar months - however she didn't know why the numbers were so different.

Another student worked out the year was 1433 - not 2012! I hadn't even noticed this little teaching gem.

The year is actually 1433! Or is it 2555 (if we were in Thailand)?

I threw in the fact that if I had bought the calendar in Thailand, the year would be 2555 - which really caused a stir. How could that be? Another girl recognised the Muslim prayer times at the bottom of the calendar -  showing how the times change throughout the month. A student with Chinese background told us the Chinese New Year was different each year and wondered if there was a connection.

By the end of the activity we had explored different number systems, how different cultures have different ideas where to start counting the year from, even how long a year is, depending if you use solar or lunar months. We shared what we knew about the words used for saying numbers in different languages - comparing those which sensibly use a simple power of ten system to those which are more complicated (try 'ninety five' in French).  Indonesia is definitely a great country for learning numbers! One student came up to the board and taught us how to do the stroke sequences to write Arabic numbers so we could improve our writing of them, then we did a few math problems with them.  After the class worked out my 12,000 rupiah purchase was actually only about $1.20, students shared their experiences of using different currencies. (Reminder to self: I must buy some of those $1,000,000,000,000,000 Zimbabwe dollar notes!)  

So many different aspects of mathematics culture embedded in just one calendar. In hindsight, I wish I had bought the entire stock of calendars at that train station. Oh well - a good excuse to return to Indonesia before the next Year 7 class.

Some teacher thoughts:

• The calendar was cost-effective, ready-to-use resource: tear off the sheets, and each table group has a large sheet with lots of space to write their ideas on.
• This resource was engaging because it contained conflicting and confusing elements - presenting a challenge for students.
• This resource should work across a wide range of ages and achievement levels - adjust your questions as needed.
• This activity is a great way for students in a culturally diverse class to share their different experiences. I was surprised how much my class taught me about the calendar - a great chance to demonstrate to how everyone can learn, even the teacher.
• At the end of the lesson, you have something to put on the wall!