The analysis process is as follows:

123 can form a number 1, 2,3.

123 can form two numbers 12,13,21,23,31,32.

123 can be composed of three digits: 123, 132, 2 13, 23 1, 3 12, 32 1.

3+6+6= 15, so: a * * * can add up to 15.

Extended data:

There are two kinds of permutation: sorted permutation and full permutation. When m different elements are taken out from n different elements, when m

That is to say, the number of permutations of all N different elements is equal to the continued product of positive integer 1 to n, and the continued product of positive integer from 1 to n is called the factorial of n, and n! Express delivery. We stipulate 0! = 1。

The calculation method of permutation and combination is as follows:

The arrangement a (n, m) = n× (n- 1). (n-m+ 1) = n！ /(n-m)！ (n is subscript and m is superscript, the same below)

Combination C(n, m)=P(n, m)/P(m, m) =n! /m！ (n-m)！ ;

take for example

A(4，2)=4！ /2! =4*3= 12

C(4，2)=4！ /(2! *2! )=4*3/(2* 1)=6