Read the first homeworks on the T on the way home Tuesday, finished them on the T on the way in today.  The math autobiographies were interesting, as usual. Only about half the class attempted the few real problems. As usual, there was little text accompanying the numbers even when the numbers were right. Of course they can’t know better yet. I posted model answers (very wordy).

The class itself was essentially a review of the material covered last time – percentages, and the 1+ trick.  I was expecting that, and did the review while answering questions about the homework due next Tuesday. In fact we did two of the problems. In the one on book markups I discovered that about a third of the class didn’t know what “wholesale price” meant. So we spent some time talking about how retail businesses work.

Several times students guessed that to find what percent A is of B you compute B/A, when A/B is right. I didn’t do a good job explaining how to avoid that error, other than to say “think, don’t try to remember a formula”.

We worked the problem that called for finding the original value of a quantity when you knew the percent increase to the new value, so I taught the technique in class – having forgotten that it’s done explicitly in the book.  I’ve sent this email to my class:

People had trouble in class yesterday figuring out last year's food
stamp usage given the percentage increase to a known value this
year. I had trouble explaining it.

In fact there's a pretty clear explanation in section 2.4 of Common
Sense. While you're reading it, fix this typo:

0.8 = 1 - 0.02

should be

0.8 = 1 - 0.2

I did get to percentage points as I’d hoped, so ready to move on to inflation next week. That will provide further review of percentage calculations and the 1+ trick.

I didn’t get to a discussion of how the 1+ trick allows you to compute relative change without having to find the absolute change first. I will add that to the book.

Still no student comments on this blog.

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