Mental arithmetics

Multiplication by 9

I recently saw a video showing a shortcut for multiplication of two-digit numbers with 9. The typical way of doing this is multiplying the number by 10 and then subtracting it from that product, like so:

36 x 9 = 36 x (10-1) = 360 - 36 = 324

In the video, however, a nice shortcut was presented:
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Multiplication with reference numbers

About the creative use of a well-known multiplication method.mere ... mehr ... more