9
Marcus then explained that numbers composed of three or more figures, transposed and subtracted in the same way, would always give a series of 9’s in the product. The children tried the experiment, and the following are some of their examples:
723
237
—-
486—4 + 8 + 6 = 18
8962
2698
——
6264—6 + 2 + 6 + 4 = 18