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The answers and rules of mathematical thinking problems in the second volume of the sixth grade of primary school.
I looked at it, and you said 20 was wrong 200; This answer is wrong; Because you think: two points can be connected into one, and three points can be combined into three. Because there were two points and two points were added, it became (1+2). Conversely, the four points are added with three points on the basis of the previous (3), so (3+3) = (1+2).