How many four digit numbers are composed of 0, 1, 2 and 3? What formula is used? Can you list these 18 numbers? Let me have a deeper understanding of this type of questions.

The principle of multiplication is used
A thousand can be three numbers, a hundred can be three, ten can be two, and one can be one
So there are 3 × 3 × 2 × 1 = 18

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