99.99×5.5－55.55×9.9 ＝99.99×5.5－11.11×5×1.1×9 ＝99.99×5.5－﹙11.11×9﹚×﹙1.1×5﹚ ＝99.99×5.5－99.99×5.5 ＝99.99×﹙5.5－5.5﹚ ＝99.99×0 ＝0