I'm glad to answer your question. This is a question of permutation and combination.

First of all, there are four ways for A to choose one of the four chairs to sit, so there are four ways. Then because this is occupied by A, there are only three free chairs for B to choose from, so there are three. Similarly, there are two ways to choose C and D, so there are two sums 1.

So now explain why it is multiplication.

Suppose A chooses seat A, then simply consider A and B, and B has three choices, then suppose A chooses seat B and B also have three choices, and so on, so only consider A and B, and there are 3+3+3 choices, which is 4*3. In the same way, you can get other situations, so use multiplication to connect them ~

I hope I have answered your question ~

Play adoption ~