In other words, three people can think of three positions from left to right.

In the leftmost position, three people can stand at first, then decide who will stand in the leftmost position, and then choose who will stand in the middle. At this time, because someone is standing in the leftmost position, there are only two choices now, and there is only one choice in the remaining position.

So it's 3*2* 1 =6.

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In fact, I don't think it's a problem for Xiaoming to ride on his father ... but I guess this question is simply made up at random.