When student enter the room, I display the XBOX One Image with the question, "Did I give you enough money to buy the Xbox One?" At this point students are wondering, "what money?" I then hand each group an envelope with a wad of cash.
The denominations I give out are Zimbabwe currency notes:
I mix up the amounts in each envelope, but they should have amounts that are either above, below or exactly 343 or 344 trillion (see below for calculated cost of Xbox One in $Zimbabwe). The idea is to give them amounts that either are or are not enough to make the purchase. I like to give exactly $Z 343 trillion, which is about $Z 312 million short of the required amount.
I ask students, "what do you need to know in order to figure out if you have enough?" Here students will recognize that the price of the Xbox One is given in US dollars. So knowing the ratio of Zimbabwe dollars to US dollars is the key. I show them this graphic and then let them count the cash in their envelopes. The graphic shows that in the year 2008, 688 Billion Zimbabwe dollars = 1 US Dollar.
This makes the problem essentially about multiplication with scientific notation:
499 US Dollars (cost of an XBox One) x 688 Billion $Z per $US
I encourage students to use scientific notation, rounding, and estimation.
(4.99 x 10^2) x (6.88 x 10^11) = 3.43312 x 10^14 = 343,312,000,000,000
This is rounded to about 343 trillion.
With Estimation (I love to encourage students to estimate, which gets them into the habit of developing instinct with numbers):
(5 x 10^2) x (6.8 x 10^11) = 340,000,000,000,000
This is 340 trillion.
Estimation is important for many reasons. I like to ask the class questions like:
The idea is that you can almost see that 5 x 6 = 30 and 5 x .8 = 4 (since 5 x 8 = 40) and thus 5 x 6.8 = 34. Then you have 34 x 10^13 = 3.4 x 10^14. I think it is amazing that we can deal with such large amounts in our head.
The next question is dependent upon the estimation and shifts the focus from multiplication to division: Is this estimation acceptable?
It is interesting because students will first notice that our estimation is over 3 trillion Zimbabwe dollars. But does this matter? To best answer that question, we can convert amounts to US Dollars.
Since 688 billion Zimbabwe Dollars = 1 US Dollars and 3 trillion $Z dollars = x $US we can solve with division:
(3 x 10^12)/(6.88 x 10^11 ) = 4.36 $US
Now we can better answer the question, "Is our estimation acceptable?"
In this part of class, students go "shopping" for other items. The question is: How much would that cost in Zimbabwe Dollars circa 2008?
I print out a variety of fun objects to "purchase" and let students pick blindly from a shopping bag. Their job is to evaluate the cost of the item they pick. The two sources I used to find these items are listed at the bottom of this section. The photos are listed there as well with prices in the name of the file.
For example, students might pick the Aston Martin with a price tag of $1,850,000.
Once students have their items, they begin their analysis. It starts by asking questions that mirror the process in the Start Up and transforms to more questions around division.
I post this question on the board along with several followup questions:
A list of "most expensive" from the web site Born Rich.
24 ridiculously expensive everyday items from the web site Cool Material.
Here the focus of activity is for students to share the cost of each item in both US and Zimbabwe dollars and then discuss the algorithms s/he used to calculate the cost. I am particularly interested in the students' ability to break down and estimate numbers. I want to know if students can find reasonable ways to estimate, and, if they can determine if their estimate is reasonable. This opens up a debate for the class.
For example, if a student has an item that cost 1,250 US Dollars, they would find the exact amount in Zimbabwe dollars by multiplying 1,250 x 6.88 Billion. However, they could estimate and solve mentally doing 2 x 6.8 Billion. The question becomes, "Would this estimate be above or below the actual amount?" We would also discuss why this might be a better estimate than "1 x 688 billion," which is typical in rounding (students see 1.25 and think "I should round down.") We could get into more detail by discussing if 2 x 6.8 billion is a better estimate than 1.3 x 6.8 billion. Here, we would talk about the impact of place value on the estimates we make.