 # Misadventures in Teaching — The Difficulty of Simple Arithmetic

## Simple Sums

I thought it would be easy for someone to follow some simple instructions and then do the simple math those instructions describe. Especially if the instructions were accompanied by an example very similar to what they had to do. I was wrong.

The class had a host number assigned to each workstation: 1, 2, 3 and so on. They were to assign a specific IP address based on that number in the first exercise, and in a later exercise, assign three addresses based that number.

Every host needs a unique network address, so the exercise manual could not tell them precisely what to use. They would have to think just a little, and in the later exercise, do some very simple math. Or at least it seemed simple.

## Exercise Steps

The exercise manual says something like the following:

The instructor has assigned host numbers to the servers in the room: 1, 2, 3 and so on. You need to set up three network addresses of the form 10.1.1.X on your server, where you replace that final X with your host number, with that number plus 100, and with that number plus 200.

For example, if your server were assigned the number 42, you would use these three addresses:
Fill in these blanks:

My number: _____

There were nine systems in use in the room, two of them with two students sharing the keyboard, the other seven with one person each. The students were mostly from high-end contractors based around Washington, D.C., providing technical services to the U.S. Department of Defense and the intelligence community.

Six of the systems managed to get their arithmetic done correctly. That's 67% if I generously round up. As for the other three:

The person on server #3 calculated:
` 3 + 100 = 3 `
` 3 + 200 = 3 `

The person on server #7 calculated:
` 7 + 100 = 17 `
` 7 + 200 = 27 `

The person on server #9 calculated:
` 9 + 100 = 100 `
` 9 + 200 = 200 `

In all three cases, I said something like "You're supposed to add 100 and 200 to your host number" and got the response "I did!"

Since then I have also seen this:
` 3 + 100 = 333 `
` 3 + 200 = 666 `

In other weeks teaching this same course I have seen people select random numbers. When you ask them what they did they say, "But I didn't think this matters!"

More recently, I saw someone simply put `3+100` and `3+200` into the file. Boot scripts don't get it when you say you want to use IP address `10.1.1.3+200`.