I looked at the calendar and the syllabus – pretty scary. Since the goal is about a chapter a week, I started Chapter 2 (units) today. I did the Larrick & Soll MPG illusion (Section 2.2 in the book). The straw poll at the start came out
- upgrade SUV 0
- upgrade sedan 20
- don’t know 1
The “don’t know” is in a sense the best answer – you need to do the numbers to find out.
We did, and discovered that the second option actually saves more gas (but not much – no one was impressed by the amount, even though they were surprised by the fact that the second choice was better).
I used the presentation as a platform for introducing unit computations using the words that name the units. They tell you when to work with miles/gallon, when with gallons/mile. (I also recommended always writing fractions with horizontal fraction bars, not slashes:
miles
——
gallon
Then the fun started. I decided to work Exercise 2.9 in Common Sense – a quote from the NY Times Green Blog reporting a Consumer Reports study saying that to pay back the extra cost of a Lexus hybrid, gas would have to cost 8.77 $/gallon. (We had an interesting few minutes talking about why units for money might precede the number when all other units seemed to follow it.) When we worked through the unit calculations, we discovered that the assertion in the article was wrong. The article should have said “$8.77/gallon more than it costs now ($2.80/gallon)”. I wonder if the error is from the original Consumer Reports finding, or in the transcription to the blog.
Good lesson: the newspaper can be wrong – you need to pay attention. I pointed out that this problem was marked [U] for untested in the text – I saw the quote and decided it would make a good exercise, but didn’t do it until today. Now I have to go back to the book and rewrite the problem. I think I will ask the students to find the mistake, not just to confirm (or not) the arithmetic. If the article was current I’d ask for a response to the blog post or a letter to the editor. I think I’ll put that in the book.
Next time – currency conversion, I think.
blog home page