Excellent resource from the Freudenthal Institute in the Netherlands, that can be used in a variety of ways. It uses java so will not work on all browsers. However there is an alternative way to get to it.
If you use Chrome see Method 2 to download this resource. Also use Method 2 if Method 1 gives an error message or raises a problem.
Method 1 for download and use
Method 2 for download and use
If videos do not work
If you use Chrome as your browser then these interactive files might not work (you are instead asked if you want to Keep or Discard the files), then Discard and go to the link here and follow the instructions to enable the relevant plugin. Mac and Windows instructions are both there: scroll down. You only have to do this once. You will need relevant permission to do this on the computer.
An alternative using online Desmos applet
Go here to see an alternative way to do this using the excellent Desmos app.
Uses of this applet
The Inquiry Maths activity: Steps Inquiry which has it’s own lesson plan (click through the Google slider slides). Using either Algebra Arrows or the Desmos applet will change the inquiry by providing multiple representations of the problem (and make some of the arithmetic unnecessary. It is highly recommended though that the inquiry is started without use of either of these applets.