2. Since the hundred digits cannot be 0, there are four options.

Ten digits can't be the same as one hundred digits at this time, so there are four choices.

You are not required to be like hundreds of dozens, so there are three ways to choose.

According to the principle of multiplication, a * * has 4*4*3=48 options?

3. There are three possibilities for one piece to be defective, and the last two pieces are genuine and not repeated.

***3*97*96=27936 species

4.( 1) If A stands in the middle and is regarded as fixed, the arrangement is equivalent to a full arrangement of six people.

* * * A6 (6) = 6 * 5 * 4 * 3 * 2 *1= 720 species.

(2) When A is on the left and B is on the right, it means that all the remaining five people are lined up, *** 120.

But Party A and Party B are interchangeable, so there are ***240 kinds.

(3) Party A and Party B must be adjacent, so both parties should be regarded as a big whole, for example, set to C.

The titles are equivalent to 720 kinds of all the other five people (***6 groups) in C. 1

There are two positions of A and B, so there are *** 1440.

5.( 1) There are 8 possibilities for each retrieval, so * * * has 8 cubes =5 12 retrieval methods.

(2) There are 8 for the first time, 7 for the second ball and 6 for the third ball, so ***8*7*6=336.

(3) C8 (3) = 56 species

6. pictures

I can't see the footprints clearly. I should bring the formula in.