This is what I learned before learning the function, laying the foundation for the function. It is equivalent to that a is x in the function, b is y in the function and f is the calculation formula, which is easy to understand. An x can only get one y, and there will be no case where an x gets two y's, but the answer from two x's is the same y, so I understand it myself. So there must be more x than y.

So this question asks you how to correspond that A has four X's and B has two Y's. It's too simple. The first x has two choices, and the second x also has two choices. Similarly, a * * * has a choice of 2 * 2 * 2 = 16, but this choice includes -65438 in four X.