To learn about Mathigon first watch the very short introductory video and see For Teachers which includes free tools, lesson plans and professional development. The professional development section includes details of upcoming free events which are held regularly and recordings of all the past event recordings.
For an introduction to Mathigon, note the session from Eric Curts, Control Alt Archive Workshop where he interviews Mathigon’s head of content David Poras. You can also read a post on Control Alt Achieve.
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Have a look at Multiplication by Heart.
The tutorial video is very clear on how this resource can be used with spaced repetition, there is also, very usefully a Shuffle mode for simple practice.
Choose the Settings and Instructions icon for a summary of the resource and access shuffle mode to practice with a random subset from all decks at once. You can also jump to a later deck with more difficult questions and different visualisations.
It is possible to embed activities – an experiemnt!