Multiply in your head, the teens

The red area is 200
The blue area is the product of the final digits.
So, for example, 13 * 17 = 200 + 21
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The same picture can be drawn for multiplication of certain two 2-digit numbers in the 20's, the 30's, etc, where the basic "red" amount is the product of the upper and the lower multiple of ten. 32*38=30*40+2*8, for example; or 1216
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If you can multiple the numbers from 1 through 19 in your head, which is not all that hard, you can then extend this same method to multiplying two numbers in adjacent "tens", where the units add to 10
like 21*39 = 20*40+1*19 = 800 + 19 = 819, and
like 23*37 = 20*40+3*17 = 800 + 51 = 851, and
like 30*30 = 20*40+10*10= 800 +100 = 900