Assignment: 07-Dividing by a Decimal
To teach decimal division, I begin by telling the story of the Frog and the Dog. the kids each create a picture like the one below:
Once upon a time there was a dog who lived in a house. He was a pretty peaceful dog, but he was also scared of just about everything. He had a new neighbor move in. His new neighbor was a frog. Now, this was a mean, mean frog. The dog quickly became scared of the frog. Soon as the frog discovered this, he started to taunt the dog. He would jump at the dogs house. Each time he would jump toward the house, the dog would jump away. Eventually, the frog gets right up next to the dog’s house, and the dog gets so scared he jumps through the roof.
The first thing we need to know about dividing by a decimal, is that you can’t. This story teaches us how to change the problem from dividing a decimal into a problem where we divide multi-digit numbers instead. For a review on that lesson, click here.
Like I said, we simply can’t divide by decimals, it’s much too difficult, and much easier to divide by whole numbers. The frog jumping towards the house represents multiplying the divisor by whatever power of 10 is necessary to make it a whole number. In the problem pictures above, you are dividing by 1.7 so you’d have to multiply it by 10 to make it 17. To keep the math correct, whatever we do to the number on the outside, we have to do to the number on the inside. This is represented by the dog jumping each time the frog does. The frog jumping through the roof is simply a reminder to put the decimal point up into the quotient as well. Here’s another example, without the frog and the dog intertwined with it:
