How many five digit numbers can be made up of six numbers 1, 2, 3, 4, 5 and 6? 6*5*4*3*2*1=720 Why should we multiply the number without repetition from 6 to 1? What is the meaning of multiplication here? Please help me understand deeply

How many five digit numbers can be made up of six numbers 1, 2, 3, 4, 5 and 6? 6*5*4*3*2*1=720 Why should we multiply the number without repetition from 6 to 1? What is the meaning of multiplication here? Please help me understand deeply

This is the problem of permutation. For example, if the first number is selected as 6, then the remaining five numbers can be permuted completely, so you can't choose 6 any more. So there are five kinds. After the second number is selected, there are four kinds. According to the multiplication principle, there are 5 * 4 * 3 * 2 * 1, and we assume that the first number is 6, which can also be assumed to be 1, 2, 3, 4, 5, 6 * 5 * 4 * 3 * 2 * 1