My last post

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I just quit my job and will not be going to back to teaching. So this will probably be my last post on this blog.

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One last math problem

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This will be my last post about teaching math. In a day or two I will explain why.

For now, here is one last problem that I made up that I like. It is to help highlight the difference between percent change and percentage points:

 

Alfred and Anna have savings accounts at The Bank of Hubert’s Left Toe (a very respectable bank). They notice that the interest rates have gone up this year.

Alfred says, “That was lame, we are only earning 1,5% more money now.”

Anna responds, “No, you are mixing up percentage points and percent change. We are actually earning 24% more money now. That’s a big change!”

Assuming Anna is right, what percent is the interest now after it has gone up?

Bad math in society

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I have always had a low tolerance for bad math out in society – if you can’t do your (often simple) job then let someone else do it. Since becoming a math teacher I find I have even less tolerance. So to the student whine of “why do I need to know this?” I would really like to answer “so you don’t look like a total moron doing the low-skilled job you thought didn’t require even basic math skills to do“.

True story number one:

A few years ago my wife was shopping at a jewelry store. They were having a necklace sale: buy 3 and get the cheapest one for free. The woman in line in front of my wife bought 6 necklaces. The cashier tried to give her the cheapest two for free. The customer argued and said that she should get a more expensive one for free. The cashier didn’t understand. The customer was quite frustrated at the stupidity and incompetence of the cashier who clearly could not do her job because she could handle the incredibly simple logical thinking required in this situation.

To illustrate for those having trouble picturing this, imaging the following size prices (in Swedish kronor):

2000, 1800, 1600, 1400, 1200, 1000

The cashier wanted to give her the 1000 SEK and the 1200 SEK necklace for free. The woman wanted the 1000 SEK and the 1600 SEK necklace for free instead. I’ll let you figure it out if you haven’t already (I even arranged the numbers nicely for you).

True story number two:

The following picture is from our local supermarket.

Image

In case you are having trouble reading the blurry text. These are 50g packages of yeast. They cost 7.90 SEK each. The price in yellow is what is the called the “comparison price”, which is given in cost per kg. Listed in this case as 15.80 SEK. Seeing the problem yet?