Details are as follows:

C=C

C=5×4×3/3×2× 1= 10

C=C=C=5×4/2× 1= 10

So the permutation combination c is equal to C.

Extended data:

Related difficulties

1. Abstracting several concrete mathematical models from different practical problems requires strong abstract thinking ability;

2. Restrictive conditions are sometimes obscure, which requires us to accurately understand the key words in the question (especially logical related words and quantifiers);

3. The calculation method is simple and has little connection with the old knowledge, but it needs a lot of thinking when choosing the correct and reasonable calculation scheme;

4, the calculation scheme is correct, often can't use intuitive method to test, requires us to understand the concept, principle, and have strong analytical ability.