Multiplication trick

If you enjoy math 'tricks', here's one presented nicely in a slide show:

How to multiply numbers with 11-19 in less than 5 seconds

It explains how to multiply by 11, 12, etc.

And... here's a challenge (you can ask this of students, too):
After watching the slideshow, explain how it actually has very similar things going on as the standard algorithm.

In other words, how do the standard multiplication algorithm (in columns) and this one compare?

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