This arrangement and combination in middle school is actually very simple, but because it is a primary school topic, the first question can be explained in a simple way, because the topic has already said that black tops can only be matched with black pants, that is to say, only two tops can be matched freely, and there are three pairs of pants that can be matched: white and black, blue and black, white and blue, blue and white, light blue, and black and black, which are exactly seven kinds of two questions, because the topic has already said that black tops can only be matched with black. Explain that only two tops and two pants can be freely matched: white, white, blue, blue and white, plus black and white. This is only the third problem of five kinds, because there is no restriction on three tops and three pants: black, white, black, blue, white, blue, blue. This is only nine kinds.