If you are multiplying two even or odd numbers, you can square their average and subtract the square of their difference from the mean to get the product.

For an easy example, 18 x 22. The square of 20 is 400. Subtract 4 (2 x 2) to get 396. Simple.

Slightly harder 21 x 13. Square of 17 is 289. Subtract 16 (4 x 4) for 273. Not simple but not hard.

31 x 19. Square of 25 is 625. Minus 36 (6 x 6) equals 589. Brilliant!

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